Quaternary Science Journal - The Palaeoecology of the Interglacial Deposits at Histon Road, Cam...

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The Palaeoecology of the Interglacial deposits at Histon Road, Cambridge

By B. W . SPARKS a n d R. G . W E S T , C a m b r i d g e

With 7 figures and 2 tables Z u s a m m e n f a s s u n g . Von den interglazialen Schichten in Histon Road, Cambridge, ist ein 8-m-Kern gewonnen worden. Er gestattete eine genaue Untersuchung der Pflanzen- und der Land- und Süßwasser-Molluskenreste.

Die betreffenden Schichten gehören der Zone g der letzten Interglazialzeit und der Zone h-i an: letzteres Symbol soll andeuten, daß, obwohl die Kontinuität der Schichten nicht unterbrochen ist, die Picea-Zone, h, in diesem einzigen in Großbritannien bekannten Ausschnitt aus diesem Teil der letzten Interglazialzeit fehlt. Von Zone / ist keine Spur gefunden worden.

Um ein Bild von der Paläoökologie zu bekommen, wurden die makroskopischen Pflanzenreste und Mollusken aus 41 Teilstücken inventarisiert, von denen jedes durchschnittlich 15 cm Länge hatte. Etwa 3000 Früchte und Samen und 16000 Mollusken wurden gefunden. Diese sind in öko­ logische und klimatische Verteilungsgruppen eingeteilt worden. Die Bedeutung der Variationen innerhalb dieser Gruppen wird behandelt. Die Pflanzenreste und Mollusken scheinen ein einheit­ liches Bild von einem Zustand zu bieten, der vom Sumpf bis zum fließenden Gewässer variiert, wie es von einem aufschüttend mäandrierenden Flusse zu erwarten ist.

R e s u m e . On a fait un sondage dans les sediments marneux interglaciaires a Histon Road, Cambridge, pour faire une analyse correlative des restes vegetaux at des mollusques terrestres et fluviatiles. C'est le gisement le seul connu en Angleterre qui comprend les zones g et h-i de la derniere epoque interglaciaire. On se sert du signe h-i, pour indiquer que la zone h, celle de Picea, manque, bien qu'il n'y ait aucune discontinuity visible dans les sediments. On n'a pas trouve la zone /, celle du Quercetum Mixtum.

Pour analyser en grand detail les changements du milieu physique, qui se produisaient pendant le depot des marnes, on a tenu compte de 41 divisions dont chaque etait d'une epaisseur moyenne de 15 cm. Environ 3000 graines et fruits et 16000 mollusques, trouves dans les marnes, ont ete divises en groupements climatiques et ecologiques et on a discute l'importance des changements de frequence de ces groupements de graines et de mollusques. Tous les deux montrent qu'il y avait une variation du milieu physique de marais riverain jusqu'ä pleine riviere tel qu'on attend d'une riviere alluvionnante qui serpente ä travers sa propre plaine alluviale.

S u m m a r y . The Last Interglacial deposits formerly exposed along Histon Road, Cambridge, have been cored and an attempt made to integrate the analyses of plants and mollusca. The beds cored are referred to zone g of the Last Interglacial and to zone h-i, a symbol used to indicate that, although there appears to be no unconformity, the Picea zone h is absent in this, the only known British section of this part of the Last Interglacial. N o trace of zone f was found.

In order to analyse the palaeoecology, the macroscopic plant remains and mollusca were recorded from 41 subdivisions, each representing on an average 15 cm of core. Some 3000 fruits and seeds and 16000 mollusca were recovered. These have been divided into environmental and climatic or distribution groups and the significance of variations of these groups discussed. The plant remains and the mollusca seem to present a consistent picture of conditions varying from riverside marsh to full stream conditions, such as might be expected from a shifting, aggrading river.

I. I n t r o d u c t i o n I t is o b v i o u s f r o m earlier a c c o u n t s ( H O L L I N G W O R T H , A L L I S O N 8C G O D W I N 1 9 5 0 , W A L K E R

1 9 5 3 ) t h a t t h e L a s t Interglacial deposits u n d e r l y i n g t h e H i s t o n R o a d , C a m b r i d g e , a r e rich in both p l a n t s a n d mollusca. U n f o r t u n a t e l y , t h e earlier of these accounts, a l t h o u g h containing a d e t a i l e d analysis of u p w a r d s of 5 0 0 0 mollusca b y K E N N A R D , does n o t c o n t a i n a complete pollen diagram in t h e light of which t h e significance of t h e mollusca m a y b e assessed, w h i l e t h e later p a p e r , a l t h o u g h r e c o r d i n g in a n almost c o m p l e t e pollen d i a g r a m t h e presence of JESSEN 8C M I L T H E R S ' zones g a n d i, listed only a f e w mollusca from t w o b r o a d horizons. A s the i n t e g r a t i o n of frequency analyses of pollen, macroscopic p l a n t

B. W. Sparks & R. G. West

124

remains a n d n o n - m a r i n e mollusca a t Bobbitshole, Ipswich ( W E S T 1957, SPARKS 1957) seemed c a p a b l e of yielding a n interesting reconstruction of n a t u r a l conditions, it w a s decided t o re-examine the H i s t o n R o a d deposits in the same w a y . Because the b e d s a r e n o longer exposed, this necessitated t a k i n g a 10 cm core t h r o u g h the deposits to a d e p t h of 8 metres. F o r their assistance w i t h the b o r i n g , w e are g r e a t l y i n d e b t e d to D r . C . L. FORBES. Mr.

C. W. PARKIN, Mr.

B. S E D D O N a n d

Dr. D. WALKER.

II. S t r a t i g r a p h y T h e following succession of deposits w a s exposed in the c o r e : 0—160 cms R u s t y y e l l o w p r e d o m i n a n t l y flint gravel, the u p p e r m o s t 20 cms b e i n g developed i n t o t h e m o d e r n soil. This u n p r o m i s i n g m a t e r i a l w a s n o t sampled.

160—230 cms P r e d o m i n a n t l y grey silty m a r l w i t h a streak of flint g r a v e l a t 170—180 cms.

2 3 0 — 2 4 0 cms G r e y - y e l l o w flint gravel.

2 4 0 — 3 0 0 cms B r o w n silty m a r l containing t o w a r d s the base m o r e p l a n t debris t h a n in a n y other p a r t of the core.

3 0 0 — 3 4 0 cms G r e y silty m a r l .

3 4 0 — 3 5 5 cms Light r u s t y - b r o w n silty s a n d .

3 5 5 — 4 9 0 cms G r e y silty m a r l becoming slightly b r o w n n e a r the base.

4 9 0 — 5 1 0 cms G r e y s a n d y g r a v e l , consisting p r e d o m i n a n t l y of flints b u t w i t h s o m e erratic m a t e r i a l , e.g. q u a r t z i t e , fragments of Gryphaea a n d p u r p l e p o r phyrite.

510—625 cms B r o w n silty m a r l .

6 2 5 — 6 4 0 cms M o t t l e d b r o w n grey silty m a r l .

6 4 0 — 7 0 0 cms Light grey silty m a r l .

7 0 0 — 8 1 0 cms W a t e r l o g g e d grey flint g r a v e l , p r o b a b l y resting on G a u l t C l a y .

T h e s e divisions are s h o w n b y the side of t h e pollen diagrams on Figs. 1 a n d 2.

T h e general succession of (c) r u s t y g r a v e l , (b) grey silty m a r l , (a) b r o w n silty m a r l , agrees well w i t h the lithological divisions of (c) s a n d a n d g r a v e l , (b) grey m u d , a n d (a) sepia m u d of W A L K E R (1953), b u t the c o r r e l a t i o n with t h e section described e a r l i e r ( H O L L I N G W O R T H , ALLISON & G O D W I N 1950) is less clear. B o t h of these earlier sections w e r e w i t h i n 100 metres of t h e present b o r i n g , which was t a k e n in t h e south-west c o r n e r of the field b e t w e e n A r b u r y R d . a n d H i s t o n R d . (very near t h e 43-foot spot h e i g h t o n H i s t o n R d . s h o w n on the 1 : 10,560 m a p , C a m b r i d g e s h i r e X L S . W . ) .

III. P o l l e n d i a g r a m s : z o n a t i o n a n d c o m p a r i s o n s T h e results of pollen analysis of the sediments are shown in Figs. 1 a n d 2, t h e first dealing w i t h the pollen of trees a n d shrubs, a n d showing the r e l a t i v e p r o p o r t i o n s of various categories of pollen a n d spores, t h e second dealing w i t h the herbaceous p o l l e n a n d the spores.

Pollen w a s present in the silty m a r l s in all t h e sequence except a t t h e base, in cores 13, 12 a n d t h e base of 11, a n d a t a f e w horizons in cores 1 a n d 2. T r e e pollen f r e q u e n c y is s h o w n in figure 1, expressed as t h e n u m b e r of 2 - c m traverses r e q u i r e d to count 100 tree pollen grains, a n a p p r o x i m a t e a n d relative m e t h o d of measurement.

T h e d i a g r a m s can be d i v i d e d i n t o t w o zones o n the basis of the tree p o l l e n : a l o w e r zone w i t h high values of Carpinus a n d Quercus pollen, a n d a n u p p e r zone w h e r e Carpi­ nus a n d Quercus are replaced b y Pinus a n d Betula. T h e b o u n d a r y between t h e z o n e s is t a k e n a t 500 cms. A l t h o u g h a s t o n y layer separates the zones, it is believed t h a t n o g r e a t u n c o n f o r m i t y exists between t h e m , because t h e vegetational t r e n d s , such as the increase of Betula a n d the decrease of Carpinus, are n o t g r e a t l y upset across t h e zone b o u n d a r y .

ON

HISTON

ROAD,

CAMBRIDGE,

1958 PERCENTAGES

Cf TOTAL TREE

POLLEN

T

IM M

iTiTm 1 1 1 1 1 R 1 1 1 1 1 1 1 1 n 1111 n I iTniTm V r ^ r ^ r ^ r r r 4 r r n ^ ° r 4 T ' T r r n r r r^-r^° M n i r^r^r-f^- rHrm

20o4

300-R=!

3504004 SOac; •

500^ 5SO-;

650-?F 700-] 750800CAULT CLAY

?

=3= SILTY MARL SAND °O CHALK PEBBLES OTHER STONES. MOSTLY FUNTS

Figure 2. Herb pollen diagram through the Histon Road deposits.

CMS - 100

ZONE

Intergiacial deposits at Histon Road (A) C o m p a r i s o n with Histon Road.

the

previous

pollen

127

analyses

from

the

"WALKER'S ( 1 9 5 3 ) pollen d i a g r a m s h o w e d t h e zones, s e p a r a t e d b y a metre of u n s a m p l e d deposit. T h e s e t w o zones a r e those identified in o u r p o l l e n diagrams. T h e continuous sampling p r o v i d e d by o u r cores shows t h a t o n l y t h e transition between t h e zones is absent in W A L K E R ' S d i a g r a m . N o trace w a s f o u n d in the recent b o r i n g of sediments from an older p a r t of t h e I n t e r g l a c i a l , a l t h o u g h H O L L I N G W O R T H , A L L I S O N SC G O D W I N

(1950)

described such a sediment from the debris from a trench a b o u t 1 0 0 y a r d s t o t h e south.

(B) C o m p a r i s o n w i t h pean mainland.

the Interglacials

of t h e n o r t h - w e s t

Euro­

W A L K E R suggested a correlation of t h e H i s t o n R o a d deposits, on t h e basis of his pollen d i a g r a m , w i t h zones g a n d i of t h e Eemian (Last) Interglacial. A sequence of pollen zones, a t o n, w a s described b y JESSEN & M I L T H E R S ( 1 9 2 8 ) on t h e basis of the pollen analysis of large n u m b e r s of E e m i a n Interglacial deposits in J u t l a n d a n d n o r t h ­ west G e r m a n y . Zone g, t h e z o n e following t h e mixed o a k forest zone / , is characterised by high values of Carpinus pollen, a n d z o n e i b y a p r e d o m i n a n c e of Pinns pollen a n d i m p o ­ verishment of the forest g r o w t h . O n t h e c o n t i n e n t these t w o zones are s e p a r a t e d b y zone h, a zone w i t h high values of Picea pollen. T h e c o m b i n a t i o n of a Carpinus z o n e , followed by a Picea, then a Pinus z o n e is at present u n k n o w n in t h e earlier interglacial periods of n o r t h - w e s t E u r o p e , a n d it seems satisfactory t o conclude t h a t the H i s t o n R o a d zones are to be c o r r e l a t e d w i t h zones g a n d i of t h e E e m i a n I n t e r g l a c i a l , even t h o u g h z o n e h, the Picea z o n e , is absent.

(C) T h e a b s e n c e

o f t h e Picea

zone

h.

Picea is a relatively l o w pollen p r o d u c e r c o m p a r e d w i t h Betula a n d Pinus (FAEGRI Sc IVERSEN 1950). T h e a p p a r e n t absence of t h e Picea z o n e in the H i s t o n R o a d d i a g r a m , though v e r y l o w percentages of Picea pollen are recorded, m a y indicate either t h a t the tree w a s absent from t h e region, the pollen a r r i v i n g b y long-distance t r a n s p o r t , or t h a t it was p r e s e n t only in small quantities. E d a p h i c or climatic factors m a y h a v e p r e v e n t e d Picea from p r e a d i n g t o t h e degree indicated by the high pollen percentages seen in the continental pollen d i a g r a m s . T h u s the Pinus zone of t h e H i s t o n R o a d deposits w o u l d a p p e a r t o b e c o n t e m p o r a r y w i t h the Picea a n d Pinus zones of the continent, a n d there­ fore t h e Pinus zone is h e r e labelled h—i.

I V . S e c o n d a r y d e p o s i t i o n of p l a n t r e m a i n s C e r t a i n changes which occur in t h e pollen analyses of t h e u p p e r cores of z o n e h—i have n o t y e t been considered. T h e increases in the pollen percentages of Quercus a n d Corylus a r e t h e most noticeable. T h e percentages of t h e l a t t e r are much h i g h e r t h a n in the l o w e r samples, reaching 1 9 5 % of t h e t o t a l tree p o l l e n . But other features a r e n o t consistent w i t h v e g e t a t i o n a l changes which m i g h t h a v e resulted from a n a m e l i o r a t i o n of the climate. A l t h o u g h Quercus a n d Corylus pollen percentages increase, t h e p r o p o r t i o n of tree t o non-tree pollen falls, a n d Betula a n d Pinus r e m a i n t h e d o m i n a n t trees. T h e u p p e r analyses also differ from the l o w e r in the more a b r u p t changes in t h e tree pollen frequency a n d in the absence of pollen a t various horizons i n cores 1 a n d 2 .

In JESSEN SC M I L T H E R S ' ( 1 9 2 8 ) scheme of the climatic development of t h e Eemian Interglacial, a middle b e d w i t h a sub-arctic flora (zone k) separated a l o w e r t e m p e r a t e phase (zones c t o i) from a n upper t e m p e r a t e phase (zones / a n d m). But for t h e reasons given a b o v e , such climatic changes m i g h t be t h o u g h t t o a c c o u n t for the fluctuations in the frequencies of Quercus a n d Corylus pollen. H o w e v e r , A N D E R S E N ( 1 9 5 7 ) h a s recently shown t h a t t h e upper t e m p e r a t e phase is n o t t o be r e g a r d e d as a p a r t of t h e interglacial, the interglacial p l a n t r e m a i n s having been d e r i v e d by r e w o r k i n g of the sediments formed

128

B. W. Sparks & R. G. West

d u r i n g the lower t e m p e r a t e phase. As ANDERSEN p o i n t s out, such r e w o r k i n g has been f o u n d in the u p p e r layers of a number of interglacial deposits, e.g. a t H o x n e ( W E S T 1956) a n d W a l l e n s e n im H i l s (THOMSON 1951), a n d it is p r e s u m a b l y caused b y changes in w a t e r level, p r o b a b l y aided b y a cold climate w i t h solifluxion.

Likewise, it is p r o b a b l e t h a t the a n o m a l o u s changes described above c a n be ascribed to t h e r e w o r k i n g of older sediments of t h e same interglacial a n d the c o n s e q u e n t redeposition of pollen. In p a r t i c u l a r the values of Corylus a n d Quercus are t h o u g h t t o be caused b y t h e r e w o r k i n g of z o n e / deposits, which a t other sites c o n t a i n high values of Corylus a n d Quercus pollen. A similar effect w a s f o u n d in the B r o r u p H o t e l Bog described b y JESSEN & M I L T H E R S (1928), w h e r e 1 3 3 % Corylus was f o u n d in the so-called u p p e r t e m p e r a t e phase, t h o u g h only a m a x i m u m of 6 4 % w a s recorded in t h e zone / deposits of the l o w e r t e m p e r a t e phase. I n this instance also t h e Corylus rise w a s accompanied b y a Quercus rise.

T h u s there is n o evidence t h a t t h e u p p e r fluctuations were a c c o m p a n i e d b y a n y climatic change. M o r e o v e r , it is impossible to say w h a t is derived a n d w h a t is n o t derived in t h e u p p e r p a r t of zone h—i. T h e difficulties which this imposes o n t h e i n t e r p r e t a t i o n of t h e u p p e r p a r t of t h e pollen d i a g r a m s will be a p p a r e n t in the later descriptions of the f a u n a , flora a n d climate.

V. Regional vegetation history (A) Z o n e

g.

D u r i n g zone g t h e temperate deciduons trees Quercus a n d Carpinus w e r e widespread.

T h e n u t of Carpinus betulus found in t h e zone g deposits suggests t h a t this was the species concerned. Pinus and Corylus w e r e also well-represented. Betula became more frequent t o w a r d s the end of the zone. L o w values of t h e pollen of Ulmus, Tilia, Alnus, Fraxinus a n d Picea indicate that these p l a y e d no great p a r t in the local v e g e t a t i o n though t h e y m a y h a v e been m o r e a b u n d a n t d i s t a n t l y .

T h e diversity of the neighbouring e d a p h i c conditions, presuming as g r e a t a v a r i e t y then as n o w , including soils formed f r o m alluvium, gravels, chalky b o u l d e r clays of v a r y i n g heaviness, Jurassic and Cretaceous clays, L o w e r Greensand a n d C h a l k , means t h a t the pollen must h a v e been derived from m a n y different plant communities, a n d this makes difficult a detailed i n t e r p r e t a t i o n of the pollen diagrams. I t is p r o b a b l e t h a t Carpinus betulus g r e w on the less h e a v y chalky b o u l d e r clay soils in t h e region, as well as on light, w e l l - d r a i n e d soils associated w i t h C h a l k t h i n l y veiled by drift. A t the present d a y , Quercus-Carpinus woods are c o m m o n l y f o u n d o n such soils in south-east E n g l a n d (M.

C H R I S T Y 1924).

T h e a m o u n t of tree pollen, h o w e v e r , is low c o m p a r e d with t o t a l of non-tree p o l ­ len, as seen in Fig. 1, a n d it appears t h a t forest w a s b y n o means d o m i n a n t . T h e frequen­ cies of such plants as Centaurea scabiosa, C h e n o p o d i a c e a e , Plantago lanceolata, P. major a n d / o r media, Poterium sanguisorba a n d G r a m i n e a e m e a n much open g r o u n d . C e r t a i n of these species, such as Centaurea scabiosa, Plantago lanceolata, P. major a n d / o r media a n d Poterium sanguisorba are characteristic of chalk grassland, such as is f o u n d a few miles to t h e south-east a t the present d a y . T h e frequency a n d variety of t h e o p e n g r o u n d p l a n t s suggest t h a t t h e forest was p a t c h y b o t h nearby a n d o n the C h a l k t o t h e south-east.

(B) Z o n e h-i.

I n spite of t h e p r o b a b l e presence of secondarily deposited pollen in t h e u p p e r p a r t of this z o n e , the m a i n characteristics of t h e vegetation are clear. In zone h-i Betula and Pinus h a v e replaced the t e m p e r a t e trees of z o n e g a n d , a p a r t f r o m a section in t h e m i d d l e of the zone, t h e l o w r a t i o of tree to n o n - t r e e pollen indicates t h a t the forest w a s sparse. T h e pollen of G r a m i n e a e rises to large frequencies a t the beginning of the z o n e ; C o m p o s i t a e percentages show a similar trend, a n d so d o the chalk grassland plants Centaurea scabiosa, Helianthemum a n d Plantago major a n d / o r media. O t h e r p l a n t s indicative of the presence

Interglacial deposits at Histon Road

129

of open g r o u n d in this zone a r e Armeria, Artemisia, Plantago maritima, Scabiosa colum足 baria a n d p e r h a p s also C a r y o p h y l l a c e a e , R u b i a c e a e a n d Tbalictrum.

I n cores 4 , 5 a n d 6 t h e ratio of tree t o non-tree p o l l e n increases to 5 0 % , c o r r e s p o n d i n g t o a decrease of t h e frequencies of G r a m i n e a e pollen a n d other herbaceous types a t these levels. I t is difficult to determine w h e t h e r this is of local or regional significance. C e r t a i n l y above this p h a s e t h e r a t i o of tree t o non-tree p o l l e n returns to v e r y l o w values. A fuller i n t e r p r e t a t i o n of t h e upper p a r t of t h e zone is p r e v e n t e d by the p r o b a b l e presence of d e r i v e d pollen.

VI. A n a l y s i s of t h e m a c r o s c o p i c r e m a i n s T o ensure t h a t t h e macroscopic p l a n t remains a n d the mollusca c a m e from e x a c t l y t h e same horizons, t h e y were both w a s h e d out from t h e same sections of t h e cores. T h e usual p r o c e d u r e w a s t o analyse each 4 0 c m core in t h r e e divisions, t h e b o t t o m half c o m p l e t e a n d the u p p e r half in t w o 1 0 c m sections giving t h e s t a n d a r d divisions, B, T B a n d T T ( T a b l e I I ) . If c o n t a m i n a t i o n occurs it will affect t h e t o p of the c o r e in the T T division.

W h e r e lithological variation is considerable t h e cores were m o r e finely divided, e. g. core 8 , as they also were near t h e t o p of the section w h e r e the grey silty marl seemed t o be involved b y frost-heaving w i t h t h e overlying g r a v e l . Cores 1 4 a n d 1 3 were u n d i v i d e d because of t h e l o w frequency of mollusca therein.

T h e washing d o w n a n d sorting were p e r f o r m e d in the same w a y as with the B o b b i t s hole material (SPARKS 1 9 5 7 ) a n d t h e same p r e c a u t i o n s taken in c o u n t i n g the mollusca.

VII. T h e g e n e r a l n a t u r e of t h e m o l l u s c a n f a u n a Although t h e r e are at least 3 4 land species, c o m p a r e d w i t h 3 3 freshwater species, freshwater mollusca are far m o r e a b u n d a n t t h a n l a n d mollusca, as m a y be seen b y c o m 足 p a r i n g figures 3 a n d 4 . But t h e r e a r e such c o n s i d e r a b l e v a r i a t i o n s through the deposit, which will be discussed below, t h a t a n y general characterisation of t h e bulk fauna from all horizons is of little use.

Curiously, o n l y t w o species, n o w extinct in B r i t a i n , were f o u n d in the core: Bitbynia leacbi v a r . inflata, which h a d n o t been recorded before, a n d Clausilia pumila, r e c o r d e d in both the p r e v i o u s accounts. Helicella crayfordensis (=Candidula radigueli B O U R G U I G NAT) was also r e c o r d e d from b o t h earlier investigations, b u t n o t r a c e of it was f o u n d in the core. T h e same is true of t w o o t h e r extinct species recorded earlier, namely Corbicula fluminalis

(HOLLINGWORTH,

ALLISON

& GODWIN

1 9 5 0 ) and

Vallonia

pulchella

var.

enniensis (SPARKS 1 9 5 3 ) . Segmentina micromphala, recorded in 1 9 5 0 , m a y indicate a sixth extinct species, b u t it is p r o b a b l y best regarded as a v a r i e t y of Segmentina nitida.

Marstoniopsis steinii, recorded by K E N N A R D in the 1 9 5 0 list as n e w t o t h e British Pleistocene, has been r e c o r d e d from the C r o m e r Forest Bed as Bythinella steinii, living as Amnicola taylori, a n d is n o w listed as Bythinella scholtzi ( E L L I S 1 9 5 1 ) . T h e extinct species a r e m a i n l y south a n d south-east E u r o p e a n forms, w h i l e C . fluminalis n o w lives in sub-tropical Africa a n d w e s t e r n Asia. A l t h o u g h cores are ideal for analysing p l a n t s a n d mollusca horizon by h o r i z o n , they m a y w e l l not i n c l u d e r a r e extinct species, which c a n m o r e readily be picked o u t in open sections.

VIII. V a r i a t i o n s i n t h e l o c a l e n v i r o n m e n t In order to assess variations in local conditions, several b r o a d groupings of species were m a d e a n d t h e percentage frequencies of these groups a t different levels p l o t t e d in graphical form (Figs. 3 , 4 a n d 5 ) .

(A) V a r i a t i o n s i n d i c a t e d b y t h e m o l l u s c a .

T h e p r i m a r y division is i n t o l a n d and f r e s h w a t e r species, each expressed as a percen足 tage of the t o t a l mollusca a n d p l o t t e d on the left h a n d sides of Figs. 3 a n d 4 .

9 Eiszeit und Gegenwart

Freshwater

species

as percentage

of to/A/ mollusca

Peresntages

of total freshwater

A Slum species

mollusca B Catholic species

C Ditch

species

D Moving

water

I 1 1 1 1 1 1 1 i I 1 Figure 3. Frequencies of freshwater mollusca; ecological groups. Dots are used where numbers are scarcely adequate for analysis.

species

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T h e f r e s h w a t e r mollusca were s u b d i v i d e d into four h a b i t a t groups, m a i n l y on the basis of BOYCOTT'S w o r k (1936). These g r o u p s are s o m e w h a t a r b i t r a r y , as m a n y mollusca are f o u n d in a w i d e r a n g e of habitats, b u t t h e relative a b u n d a n c e of the different groups should give a reasonable indication of local conditions.

T h e first g r o u p , A, t h e slum species as t h e y were g r a p h i c a l l y described b y BOYCOTT, includes seven species, Lymnaea truncatula, Aplexa hypnorum, Planorbis leucostoma, Sphaerium lacustre, Pisidium casertanum, P. personatum, a n d P. obtusale. O f these, Planorbis leucostoma a n d Pisidium obtusale a r e the t w o which most affect t h e percentage frequency of the g r o u p a n d they are b o t h characteristic of small closed w a t e r bodies subject to periods of d r o u g h t . Some of t h e other species in this group will tolerate such conditions t h o u g h they d o n o t necessarily prefer them. Lymnaea truncatula m a y spend as much t i m e o n m u d a t t h e edge of the w a t e r as in the w a t e r a n d is regarded b y BOYCOTT as n o t r e a l l y a freshwater species.

T h e second g r o u p , B, t h e catholic species, has a wide t o l e r a n c e of habitats a n d includes ten species, Lymnaea palustris, L. peregra, Planorbis albus, P. crista, P. contortus, Segmen­ tina complanata, Sphaerium corneum, Pisidium milium, P. subtruncatum a n d P. nitidum.

Most of t h e species m a y i n h a b i t small bodies of w a t e r such as cattle ponds a n d also good localities in streams, t h o u g h they are n o t usually f o u n d in t h e environments tolerated b y the slum species of g r o u p A. It is significant t h a t t h e y a r e found in smaller numbers a n d m o r e evenly d i s t r i b u t e d through t h e various h o r i z o n s t h a n most of t h e species included in t h e o t h e r g r o u p s . Planorbis crista seems t o s h o w greater fluctuations of frequency t h a n the others.

T h e t h i r d g r o u p , C, t h e ditch species, p r e f e r quietly m o v i n g w a t e r a n d a rich g r o w t h of w a t e r p l a n t s . Seven species are included, Valvata cristata, Planorbis planorbis, P. vorticulus, P. vortex, Segmentina nitida, Acroloxus lacustris, a n d Pisidium pulchellum.

The last h a r d l y affects the frequency and of t h e others the most doubtful is Planorbis planorbis which has a w i d e r a n g e of tolerance. T h e t w o species of t h e subgenus Planorbis, h o w e v e r , prefer f a i r l y spacious h a b i t a t s of moving w a t e r a n d P. planorbis is said t o occur more c o m m o n l y in ponds a n d ditches than the o t h e r species, carinatus (BOYCOTT 1936). T h e shells listed as Planorbis (Planorbis) sp. a r e included here as t h e y are p r o b a b l y P. planorhis a n d n o t P. carinatus, which has not been recorded from t h e deposit in a n y list.

T h e last g r o u p , D , prefers moving w a t e r a n d m o r e spacious habitats a n d includes eight species, Valvata piscinalis, Bithynia tentaculata, B. leachi var. inflata, Lymnaea stagnalis, Physa fontinalis, Pisidium amnicum, P. henslowanum a n d P. moitessierianum.

Most of t h e changes in frequency of the g r o u p are caused b y increases a n d decreases in the n u m b e r s of Bithynia tentacular Planorbis laevis has n o t been i n c l u d e d in a n y g r o u p . A t present it is charac­ teristic of l o w elevation m o u n t a i n lakes in the n o r t h w e s t e r n p a r t s of Britain (BOYCOTT 1936), b u t it, o r a species indistinguishable from it, is v e r y c o m m o n in some Pleistocene deposits, for e x a m p l e Selsey, where it c e r t a i n l y does n o t i n d i c a t e low elevation m o u n t a i n lakes. F u r t h e r m o r e , there is d o u b t about m a n y of the identifications, as v e r y b r o k e n or very juvenile specimens a r e very difficult t o distinguish f r o m Planorbis planorbis and P. carinatus, so t h a t t h e y a r e recorded w i t h a query in T a b l e I I .

T h r e e groups of l a n d mollusca h a v e been separated. T h e first contains species which might be expected to live in marshes. T h e s e are m o s t l y i n c l u d e d in BOYCOTT'S (1934) obligatory hygrophiles a n d commonly associated species. F o r this deposit t h e y are Carychium minimum, all species of Succinea, Cochlicopa lubrica, Vertigo pygmaea, V• moulinsiana, V. angustior, Vallonia pulchella, Hyg*omia hispida, Punctum pygmaeum.

Euconulus fulvus, Retinella radiatula, Zonttoides nitidus a n d all species of Agriolimax.

T h e o n l y differences b e t w e e n this and BOYCOTT'S groups a r e t h e substitution of Vallonia pulchella for V. excentricia a n d the inclusion of all species of Agriolimax, because of the 9 «

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133

difficulty of identifying the shells of these slugs w i t h certainty. S o m e of the species, n o t a b l y Vertigo pygmaea, are n o t necessarily marsh species a n d m a y be f o u n d in much d r i e r habitats.

T h e d r y l a n d g r o u p (Fig. 4) includes four species, Pupilla muscorum, Vallonia costata, V. excentrica a n d Helicella itala. O f these the first a n d the last are p r o b a b l y the m o s t characteristic xerophiles, for Vallonia costata has been recorded in q u i t e d a m p localities ( E L L I S 1940), a l t h o u g h it is not characteristic there.

T h e total of all t h e species of Vallonia is also separated, for, a l t h o u g h pulchella is u s u a l l y found in much d a m p e r localities t h a n the o t h e r t w o species, n o n e are n o r m a l l y f o u n d in woods. T h u s , the frequency of Vallonia m a y be used to suggest locally o p e n environments.

U s i n g these v a r i o u s groups of mollusca, it is possible to define the v a r i o u s changes in local conditions t h r a u g h the deposit. T h e following phases m a y be recognised.

C o r e 14 is in g r a v e l at the base of the section a n d there is some d o u b t whether it is c o m p l e t e l y free from c o n t a m i n a t i o n , as the borehole w a s n o t cased to this level. H o w e v e r , t h e s t r e a m conditions suggested b y t h e lithology are confirmed by t h e p r e d o m i n a n c e of f r e s h w a t e r mollusca a n d by the i m p o r t a n c e of the ditch a n d moving w a t e r groups a m o n g them.

C o r e 13 is in g r e y silty marl, g r a v e l l y at the base a n d poorly fossiliferous. L a n d a n d f r e s h w a t e r species a r e present in a l m o s t equal n u m b e r s , the slum g r o u p of freshwater mollusca and the d r y l a n d group d o m i n a t i n g . But t h e numbers are t o o small for satisfac足 t o r y analysis a n d it is perhaps b e t t e r t o regard this core as a transition between stream c o n d i t i o n s below a n d d r y land conditions above in c o r e 12.

C o r e 12 is d o m i n a t e d by l a n d mollusca a n d represents the m o s t t r u l y terrestrial conditions revealed b y the whole succession. T h e percentage of m a r s h species is l o w e r t h a n t h e percentage of d r y land species, while Vallonia is more i m p o r t a n t a t this h o r i z o n t h a n a t a n y other level. All three species are i m p o r t a n t ( T a b l e I I ) , so t h a t , although open conditions are i n d i c a t e d , they r a n g e d from d a m p to d r y . T h e open q u a l i t y of the e n v i r o n 足 m e n t w a s p r o b a b l y local, for small o p e n places, some d a m p a n d some d r i e r and p e r h a p s a foot or t w o a b o v e flood level, w o u l d be sufficient to account for t h e frequency of Vallonia. T h e p r o p o r t i o n of f r e s h w a t e r species is too small to be s e p a r a t e d into groups, b u t species of Valvata a n d Bithynia, which are included in groups C a n d D , m a y indicate v e r y occasional flooding of land a d j a c e n t to a stream.

C o r e 11 B is essentially a t r a n s i t i o n to marsh, a f a c t which m a y also be indicated b y the transition t h r o u g h a mottled l a y e r from grey to b r o w n silty m a r l a n d b y the presence of w h a t a p p e a r t o be ferruginous nodules replacing roots a n d stems of plants. T h e p e r c e n t a g e of l a n d mollusca decreases somewhat, w h i l e the marsh a n d associated species b e c o m e much m o r e i m p o r t a n t t h a n t h e d r y land species a n d Vallonia. T h e marsh percen足 tage should p r o b a b l y be higher, as t h e r e is a large n u m b e r of specimens of Carychium, which cannot be d e t e r m i n e d specifically. As C. minimum, the marsh species, is about four times as common as C. tridentatum, it is likely t h a t m a n y of these unidentifiable shells a r e C . minimum. T h e increase in t h e freshwater mollusca is mainly in t h e slum g r o u p , so t h a t t h e transition to marsh is i n d i c a t e d by b o t h l a n d a n d freshwater groups. M a r s h conditions are b e t t e r developed a t this a n d i m m e d i a t e l y succeeding levels t h a n a t a n y o t h e r horizon except perhaps cores 4 a n d 3B. T h a t p a r t of t h e succession from core 11TB t o 8 B T B inclusive m a y best be r e g a r d e d as a slow transition. Lithologically it is all b r o w n silty m a r l grading i n t o gravel at the t o p of 8B a n d in 8 B T B , although 1 0 T T is gravel a n d so is 9 T T , which is n o t included in t h e analysis as it seemed t o be c o n t a m i n a t e d . T h e w h o l e succession represents the g r a d u a l change from m a r s h t o stream conditions. I n 11TB t h e freshwater species are d o m i n a t e d b y t h e slum g r o u p a n d it is highly likely t h a t the frequency of these s h o w n on Fig. 2 is t o o

134

B. W. Sparks & R. G. West

low, for most of the shells listed as Pisidium sp. are p r o b a b l y juveniles of P. obtusale which is v i r t u a l l y the o n l y species of Pisidium present a t this level. If this is so, the percentage of slum species should be a p p r o x i m a t e l y 5 0 % higher, as high in fact as in core I I B . A b o v e this level t h e r e is a general decrease in the i m p o r t a n c e of slum species, until b y core 8BTB they a r e of n o great significance. T h e catholic group rises t o a b r o a d peak in cores 10 a n d 9 a b o v e t h a t of the slum g r o u p a n d t h e n decreases u p w a r d s . T h e ditch g r o u p reaches a p e a k a t a slightly higher level, from a b o u t 1 0 T T to 8B, a n d then falls, while the m o v i n g w a t e r g r o u p increases the whole w a y u p and is highest in 8BTB. T h u s , there is a successive d o m i n a n c e of t h e four groups, i n d i c a t i n g the progression from marsh t o stream conditions. T h e first a p p e a r a n c e near the t o p of this p a r t of t h e core of Pisidium amnicum a n d P. henslowanum is confirmation of t h e establishment of stream conditions. U n f o r t u n a t e l y t h e land mollusca a r e too few in number for a continuous analysis t o be m a d e t h r o u g h this section. T h e i r general decrease in i m p o r t a n c e is clear a n d to be expected from t h e changes indicated b y the f r e s h w a t e r species. T h e beginning of the decrease in marsh species m a y be seen in core 9, a n d w h e n there are n e x t enough for analysis, a d m i t t e d l y in core 8 B T T just a b o v e the levels discussed, d r y l a n d species are much more i m p o r t a n t t h a n marsh species.

Cores 8 B T T to 5 T T inclusive are composed principally of light g r e y - b r o w n o r grey silty m a r l s . T h i s p a r t of t h e core is characterised b y a higher percentage of l a n d mollusca t h a n t h a t f o u n d in the section just described, a n d b y the d o m i n a n c e of m o v i n g w a t e r and catholic groups a m o n g the feswhater species. In the freshwater mollusca the change from 8BTB to 8 B T T is quite s h a r p (Fig. 3). T h e m o v i n g w a t e r g r o u p remains d o m i n a n t , while the ditch g r o u p falls to an extremely low p r o p o r t i o n . I m m e d i a t e l y above the base in 8TB the catholic g r o u p suddenly increases. T h e dominance of m o v i n g w a t e r a n d catholic groups is upset in cores 6 T B , 6 T T a n d 5B b y a subsidiary p e a k in the slum g r o u p . A p a r t from this, the assumption of generally o p e n stream conditions would a c c o u n t for the g r e a t l y decreased i m p o r t a n c e of those species preferring weed-filled ditches, a n d also for the change in the character of the land mollusca. T h e latter increase in i m p o r t a n c e a n d are d o m i n a t e d by d r y land species almost exclusively Pupilla muscorum.

In p r e v i o u s com足 ments (SPARKS in W A L K E R 1953) it was suggested t h a t this m i g h t have been d u e t o the v a r i e t y elongata CLESSIN, which is said to be characteristic of d a m p e r localities, b u t this is p r o b a b l y n o t the true e x p l a n a t i o n . T h e a p e r t u r e s of d e a d shells easily become plugged w i t h dirt a n d t h e shell thus floats freely, so t h a t d e a d shells m a y be washed into a stream from d r y habitats a n d redeposited much f a r t h e r d o w n the s t r e a m . In fact the presence of P. muscorum, r a t h e r t h a n a m a r s h fauna, seems to indicate t h e increased i m p o r t a n c e of stream activity.

T h e next recognisable p h a s e is represented b y cores 4B to 3B inclusive a n d indicates a reversion to m a r s h y conditions. T h e overall p r o p o r t i o n of l a n d snails decreases, b u t the sudden increase in the m a r s h a n d associated species a n d the corresponding decrease in the d r y l a n d species is v e r y striking (Fig. 4). E q u a l l y striking a m o n g the freshwater mollusca is t h e a b r u p t fall in m o v i n g w a t e r species a n d the spectacular increase in slum species (Fig. 3), t h e latter being especially m a r k e d b y the increase of Planorbis leucostoma ( T a b l e I I ) . T h e lithology changes to a light b r o w n m a r l , which becomes much d a r k e r in 3B. This d a r k b r o w n silty m a r l extends up to c o r e 3 T T , w h e r e , however, the r e l a t i v e fre足 quency of the groups of mollusca is v e r y different. T h e m a r s h y conditions represented are c o m p a r a b l e w i t h those of core 11TB a n d w e r e p r o b a b l y best developed in 3B, which has the highest p e r c e n t a g e of marsh l a n d snails as well as a significant p r o p o r t i o n of freshwater species liking weed-filled ditches.

T h e general composition of the fauna changes a b r u p t l y in core 3 T B a n d remains v i r t u a l l y the same to the t o p of the section. T h e percentage of land mollusca is lower t h a n a t a n y o t h e r levels a n d drops to n o t h i n g in cores 1 T B a n d I T T . T h e r e a r e far too

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few to analyse t h e m into g r o u p s . Among t h e freshwater mollusca the moving w a t e r g r o u p increases e n o r m o u s l y a n d t h e other groups d r o p to very l o w levels. In fact, t h e r e is a reversion to s t r e a m conditions. It is curious t h a t the changes d o n o t coincide e x a c t l y w i t h lithological changes. Grey silty marl, such as characterised t h e stream phase from 8 B T T t o 5 T T , r e t u r n s in core 2 B T , b u t the f a u n a l change is a b o u t a foot lower in core 3 T B .

T h e top t w o cores are i n v o l v e d by c r y o t u r b a t i o n in the o v e r l y i n g gravel, b u t all the mollusca seem to come from t h e grey m u d .

(B) V a r i a t i o n s i n d i c a t e d b y t h e

plants.

T h e p l a n t remains are listed in T a b l e I I I i n t o three ecological groups, which a r e gra足 phed in Fig. 5 First, aquatic species, such as Potamogeton spp., Sagittaria and Zannichellia; n o distinction has been m a d e between v a r i o u s categories of w a t e r plants, a n d all these plants are m o s t c o m m o n l y f o u n d in w a t e r bodies of v a r y i n g d e p t h s . T h e second g r o u p includes p l a n t s characteristic of habitats of v a r i a b l e wetness f o u n d a r o u n d the edges of w a t e r bodies, such as Alisma plantago-aquatica a n d Rumex maritimus, and the third includes p l a n t s of open g r o u n d , waste and grassy places, usually d r y . T h e t w o r i g h t h a n d columns of Fig. 5 show t h e frequencies, as percentages of t h e total at each h o r i z o n of columns 1, 2 a n d 3, of Ranunculus-Batrackium spp., a species aggregate occupying h a b i t a t s v a r y i n g b e t w e e n groups 1 a n d 2, and of Urtica dioica, w i t h habitats v a r y i n g between groups 2 a n d 3. Trees a n d shrubs are n o t included in t h e g r o u p i n g because of their infrequency. T h e percentages a r e only g r a p h e d w h e r e sufficient numbers of p l a n t remains were found.

In cores 14, 13 and 12, pollen was absent a n d macroscopic p l a n t remains were scarce.

T h e molluscan fauna in these cores shows a transition from s t r e a m conditicns in core 14 to d r y l a n d conditions in core 12. In cores 13 a n d 12 the most frequent of the l o w total of seeds are those oiRanunculus-Batrachium spp., a result a c c o r d i n g with the suggestion of terrescrial conditions w i t h very occasional flooding given b y t h e mollusca.

T h e mollusca show d u r i n g cores 11 t o 8 a transition from d r y land conditions in core 12, t h r o u g h marsh in core 11 to s t r e a m conditions in core 8. A similar change is indicated b y t h e frequencies of the groups of p l a n t remains. I n cores 11 to 9 p l a n t s characteristic of shallow w a t e r and d a m p g r o u n d (group 2), including Alisma plantagoaquatica, Oenanthe aquatica, Rumex maritimus a n d also Ranunculus-Batrachium spp. a r e p r e d o m i n a n t , with their frequencies s l o w l y falling. I n core 8 the aquatic species (group 1), including Potamogeton spp., Sagittaria a n d Schoenbplectus, become p r e d o 足 minant. T h e change in local w a t e r conditions is accompanied b y the first a p p e a r a n c e of pollen on t h e change from d r y land to m a r s h conditions from core 11 B to 11 T B . I t is interesting t o n o t e that a l t h o u g h the m a c r o s c o p i c remains s h o w these local changes in enviroment, t h e pollen of the marsh and a q u a t i c plants, as seen in Fig. 2, does n o t show a similar t r e n d . P r e s u m a b l y the pollen rain w a s n o t so locally derived.

In cores 8 a n d 7 the a q u a t i c species are d o m i n a n t , and, as w i t h the mollusca, the total number of specimens found is much reduced. T h e analysis of mollusca from the c o r r e s p o n 足 ding levels shows the d o m i n a n c e of moving w a t e r a n d catholic species.

In core 6 t h e group 2 species regain their p r e d o m i n a n c e , b u t specimen totals h e r e are far too small for much significance to be attached to the change. It does, h o w e v e r , correspond t o a n increase in the slum species of mollusca, a change which w o u l d be expected t o a c c o m p a n y the increase of the g r o u p 2 species of p l a n t s .

T h e scarcity of plant r e m a i n s in cores 5 a n d 4 prevents representation in the frequency groups. T h i s p e r i o d corresponds to an increase in the frequency of the l a n d mollusca, though of t h e freshwater mollusca moving w a t e r species a r e d o m i n a n t in core 5 a n d slum species in core 4. Urtica dioica is the p l a n t m o s t frequently represented in core 4, as it is in core 3. A t t h e base of core 3, the group 2 species, w i t h Urtica dioica, p r e d o m i n a t e , a n d

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137

this phase is c o r r e l a t e d w i t h the high frequencies of the slum a n d ditch species of mollusca, as it w a s in cores 11 to 9. T h e great frequencies of Urtica dioica here, together w i t h the records of Ranunculus sceleratus, suggest the presence of shallow m u d d y w a t e r , w i t h nearby riverside h a b i t a t s disturbed by animals. A t the t o p of core 3 a n d in core 2, the aquatic p l a n t s of g r o u p 1 (almost entirely Sagittaria, b u t also Potamogeton spp.) become more frequent, a change corresponding to the g r e a t increase in m o v i n g w a t e r mollusca seen a t this level. T h e a b u n d a n c e of Sagittaria indicates slow m o v e m e n t of the w a t e r over a m u d d y s u b s t r a t u m . Macroscopic p l a n t r e m a i n s were n o t f o u n d a t any h o r i z o n above core 2 T B , t h o u g h pollen w a s present in places. I t is n o t clear h o w far d e r i v a t i o n affects the i n t e r p r e t a t i o n of the p l a n t remains from these u p p e r cores.

T h e l a n d species, g r o u p 3, show a g r a d u a l b u t irregular increase from c o r e 11 to core 7. T h e p e a k in core 9 is caused by a t e m p o r a r y a b u n d a n c e of the seeds of Chenopodium section Pseudoblitum, a n d corresponds t o t h e peak in t h e pollen of C h e n o p o d i a c e a e a t the same level. Species of this section of Chenopodium a r e characteristic of waste ground, a n d t h e y m a y be frequent near the sea. T h e finds of Rorippa islandica a t the same level as Chenopodium subsection Pseudoblitum are of interest, as Chenopodium rubrum, the m o s t c o m m o n British species of the section Pseudoblitum, is often f o u n d with Rorippa islandica ( a n d Bidens cernuus, a species recorded f r o m the same level) in moist places by streams w i t h s t a n d i n g w a t e r in w i n t e r , a h a b i t a t agreeing well w i t h t h e condi­ tions suggested by the mollusca. T h e relatively high frequencies of group 3 species in cores 8 a n d 7 a r e caused b y the presence of few specimens of a n u m b e r of species, inclu­ ding Anthriscus sylvestris, Chenopodium sp., Cirsium cf. vulgare, Euphorbia cyparissias agg., Einum anglicum, Pastinaca sativa, Petroselinum segetum, Picris hieracioides, Plan­ tago major, Polygonum aviculare agg. and Sonchus arvensis. T h e increase in this g r o u p of species p r e s u m a b l y corresponds to the increase of the n o n - t r e e pollen, n o t a b l y of G r a m i ­ neae, C o m p o s i t a e a n d also Centaurea scabiosa, as the transition from zone g to z o n e h—i takes place.

T h e a b u n d a n c e of l a n d species in cores 7 a n d 8 corresponds t o the high frequency of the d r y l a n d mollusc Pupilla muscorum, b u t the latter c o n t i n u e in core 6, w h e r e no group 3 species are found.

I t will be seen t h a t the g r o u p 3 species reach a m a x i m u m a t the same time as t h e g r o u p 1 species; it is possible t h a t a n explanation similar to t h a t given for the a b u n d a n c e of Pupilla muscorum u n d e r stream conditions m a y a p p l y , i.e., t h a t p l a n t remains f r o m d r y habitats a r e m o r e likely to be w a s h e d d o w n a n d deposited w i t h the remains of aquatic plants u n d e r conditions of r u n n i n g water t h a n if the stream is sluggish, w h e n the more local elements of the flora will predominate.

In cores 3 a n d 2 the g r o u p 3 species are scarce, the locally g r o w i n g aquatics a n d d a m p ground elements of the flora being dominant, as w i t h the mollusca.

(C)

Conclusions.

T h e succession of e n v i r o n m e n t s indicated b y the f a u n a a n d flora is f r o m stream conditions in core 14, to d r y l a n d in core 12, t o marsh in core 1 1 , a slow change back to stream conditions b y core 8, a continuation of these conditions t o core 5, a reversion to marsh in cores 4 a n d 3B, a n d a n equally sudden change back to stream conditions above.

These changes m i g h t be e x p l a i n e d p a r t l y by a m e a n d e r i n g s t r e a m swinging across its o w n floodplain, t h o u g h the relative rates of deposition a n d m e a n d e r shift are too imperfectly k n o w n for it t o be decided w h e t h e r there are sufficient s e d i m e n t a r y or f a u n a l „cycles" for this e x p l a n a t i o n to a p p l y . T h e thickness of t h e sediment a n a l y s e d , some 20 feet, must indicate a g g r a d a t i o n , t h o u g h w h e t h e r due t o a rising base-level or to an increase in the load-discharge r a t i o in the stream is uncertain. T h e rising base-level seems u n l i k e l y in view of the fact t h a t during t h e time of the conifer pollen zones in the latter half of the

138

B. W. Sparks & R. G. West

Eemian Interglacial on the c o n t i n e n t the Eem Sea w a s undergoing regression (G. v. D .

B R E L I E 1954). T h e r e is no indication from the mollusca of salt or brackish w a t e r c o n ­ ditions a t a n y level. N e i t h e r do the p l a n t s give a n y c e r t a i n indication of such conditions, t h o u g h p e r h a p s some are suggestive of maritime c o n d i t i o n s not too far a w a y , e.g. Apium graveolens in core 10, Chenopodium section Pseudoblitum in cores 10 a n d 9, a n d the pollen of Armeria (more Armeria pollen was recorded b y W A L K E R 1953) a n d of Plantago maritima, the f o r m e r in core 5, the l a t t e r in cores 9, 8, 7, 6 and 5.

I X . V a r i a t i o n s i n c l i m a t e t h r o u g h t h e deposit (A) V a r i a t i o n s

indicated

by

the

mollusca.

In the analysis of the Bobbitshole mollusca (SPARKS 1957), it w a s f o u n d possible t o p l o t the frequencies of individual species, as well as groups of species, because the local conditions changed slowly a n d to a f a r smaller degree t h a n in this deposit. T h e p l o t t i n g of i n d i v i d u a l frequencies through the H i s t o n R o a d deposits is pointless, because of t h e a b r u p t changes in local conditions. T h i s m a y be a p p r e c i a t e d , for e x a m p l e , b y considering the a b r u p t d r o p in Valvata cristata a n d Planorbis leucostoma between cores 3B a n d 3 T B a n d the even m o r e a b r u p t rise of Bithynia tentaculata a t the same level. Y e t these t h r e e species are all h i g h l y tolerant climatically.

Accordingly all species, the distributions of which can be o b t a i n e d w i t h some cer­ t a i n l y from G E Y E R (1927), EHRMANN (1933) a n d H U B E N D I C K (1947), h a v e been classed into four groups depending largely on their range n o r t h w a r d s into i n t e r i o r S c a n d i n a v i a .

These groups are indicated in the first column on T a b l e I I . Species classed g r o u p A reach well to the n o r t h , almost to or w i t h i n the Arctic Circle in Sweden. G r o u p B reaches a p ­ p r o x i m a t e l y 63° N . in Sweden, while g r o u p C reaches 6 0 — 6 1 ° N . , i.e. a p p r o x i m a t e l y t h e n o r t h e r n limit of the oak in Sweden. Species in g r o u p D reach o n l y t h e v e r y south of S c a n d i n a v i a or are confined to the E u r o p e a n m a i n l a n d . There is a c e r t a i n a m o u n t of d o u b t concerning some of the distributions: with t h e slug genus Agriolimax and with some species of Pisidium because of identification difficulties; w i t h Limax sp. because, altough most species w o u l d be classed as group B, t h e actual species is n o t k n o w n ; w i t h Planorbis laevis, because, in spite of BOYCOTT'S i n d i c a t i o n of a n o r t h - w e s t e r n distribution in low-level m o u n t a i n lakes in B r i t a i n , it does not seem t o reach v e r y f a r n o r t h in S c a n ­ dinavia.

I t is evident from Fig. 6, as it w a s from the Bobbitshole material, t h a t the most t o l e ­ r a n t species d o m i n a t e even in the m i d d l e of an interglacial, but the p r o p o r t i o n of the m o s t tolerant g r o u p , A , does increase steadily up through the deposits as the climate d e t e r i o r a ­ tes. T h e low frequencies in cores 12 a n d 11, except in I I B , are due to the large n u m b e r of specimens of Vallonia, which c a n n o t be determined specifically. In I I B the l o w frequency of g r o u p A a n d the high frequency of g r o u p B are due t o the large n u m b e r s of Carychium, which w a s a p p a r e n t l y favoured b y local conditions. G r o u p B, a p a r t f r o m t h e d r o p f r o m the p e a k in I I B , does not begin t o decrease greatly u n t i l core 3 T B : in fact it shows a s e c o n d a r y m a x i m u m in cores 6 to 3B. W h e t h e r this is c o m p a r a b l e w i t h the i n t e r m e d i a t e m a x i m u m on the less tolerant g r o u p a t Bobbitshole (SPARKS 1957, Figs. 1 a n d 2) is u n c e r ­ t a i n , as the same species are n o t i n v o l v e d . It m a y be merely an effect of local e n v i r o n ­ m e n t a l conditions. G r o u p C is a t a m a x i m u m near the b o t t o m and decreases steadily f r o m core 6 u p w a r d s , w h i l e group D a l m o s t disappears a b o v e the middle of c o r e 8.

T h e frequencies of these groups, which are based solely on present distribution of the species, clearly shows t h a t the general climatic t r e n d m a y be derived f r o m a frequency analysis of mollusca species or g r o u p s , in spite of t h e complicating effects of changes in local conditions. T h e virtual d i s a p p e a r a n c e of the least tolerant g r o u p , D , a t the e n d of pollen zone g is interesting, though it m a y be safer t o r e g a r d it as a coincidence until m o r e

139

Interglacial deposits at Histon Road

Climatic groups A T B

1

BT

I

2 3 I

5 6 7

ZO/Vf /)-/'

81 9 T

10 ^ li 12

13 14-

I I

Figure 6. Frequencies of mollusca; climatic groups. Dots are used where numbers are scarcely adequate for analysis. d a t a f r o m other deposits are available. I t seems t h a t t h e less tolerant mollusca linger on after t h e m i d d l e of t h e interglacial i n t o zones with m o r e n o r t h e r n v e g e t a t i o n , e.g., the pine-birch period of z o n e h—i, t h a n t h a t characteristic of the vegetation zone in which t h e y w e r e i n t r o d u c e d , for m a n y of t h e species in g r o u p s B a n d C w e r e n o t really i n t r o ­ duced u n t i l zone e, t h e birch-pine-oak z o n e a t Bobbitshole. This relationship might h a v e been expected, for i m m i g r a t i o n in the first half of a n interglacial must d e p e n d on r a t e of m i g r a t i o n as well as climatic i m p r o v e m e n t , while e x t i n c t i o n in the second half is p r o ­ b a b l y p r i n c i p a l l y a function of c l i m a t e . T h e i n t e r p r e t a t i o n of a fauna a t a given level, then, will depend p a r t l y on k n o w i n g which p a r t of t h e interglacial it belongs to. A t the t o p of t h e section t h e complete d o m i n a n c e b y very t o l e r a n t British f r e s h w a t e r species is c o m p a r a b l e with c o n d i t i o n s in the e a r l y p a r t s of the interglacial, for e x a m p l e at the b o t ­ t o m of the Bobbitshole section.

Interglacial deposits at Histon Road (B) V a r i a t i o n s

indicated

by

the

141

plants.

T h e plants r e c o r d e d by their macroscopic r e m a i n s w e r e assigned w h e r e possible t o five p l a n t - g e o g r a p h i c a l groups, as s h o w n in T a b l e I I I . T h e s e categories of distribution a r e those which were used in the analysis of the p l a n t r e m a i n s from Bobbitshole ( W E S T 1 9 5 7 ) , a n d a r e m a d e for use w i t h H U L T E N ' S ( 1 9 5 0 ) Atlas of t h e distribution of vascular p l a n t s in N . W . E u r o p e . T h e y a r e : 2 . Plants distributed throughout Scandinavia.

3 . P l a n t s w i t h n o r t h e r n limits in S c a n d i n a v i a n e a r the Arctic C i r c l e .

4 . P l a n t s w i t h n o r t h e r n limits in S c a n d i n a v i a a b o u t m i d w a y b e t w e e n those categories 3 a n d 5 .

5 . P l a n t s w i t h n o r t h e r n limits in the south of S c a n d i n a v i a .

6 . P l a n t s w i t h n o r t h e r n limits in north-west E u r o p e south of S c a n d i n a v i a .

in

F i g u r e 7 shows t h e percentage frequency of specimens of species of each g r o u p h o r i z o n b y h o r i z o n . I d e a l l y a n analysis of this k i n d should be expected to r e v e a l long t e r m cli­ m a t i c fluctuations (here the analysis is „ n o r t h - s o u t h " , b u t an „east-west" or oceanicc o n t i n e n t a l analysis o r a n analysis of t h e effects of climatic change on t h e different ecolo­ gical g r o u p s m i g h t be possible in o t h e r deposits).

I n the H i s t o n R o a d deposits, h o w e v e r , the i n t e r p r e t a t i o n of the frequency g r a p h is complicated b y t h e possibility of s e c o n d a r y deposition in the u p p e r cores, a n d b y the small n u m b e r s or absence of macroscopic remains in t h e m i d d l e cores.

A general t r e n d of change in distribution t y p e is a p p a r e n t . I n the l o w e r cores, p l a n t s of g r o u p s 3 a n d 5 a r e most a b u n d a n t , a n d in the u p p e r cores, plants of groups 2 a n d 3 .

T h e s e changes are similar to those s h o w n b y the mollusca a n d record t h e deterioration of climate in the latter half of the interglacial. In the Carpinus zone g, t h e r e is a large p r o ­ p o r t i o n of specimens of species n o w o n l y reaching n o r t h w a r d in S c a n d i n a v i a as far as the s o u t h e r n half of S w e d e n , a group including also c e r t a i n p l a n t s identified b y their pollen, e.g. Quercus, Carpinus, Corylus, Hedera, Ilex, Centaurea scabiosa a n d Poterium sanguis­ orba. T h e p r e d o m i n a n c e of these t h e r m o p h i l o u s p l a n t s , together w i t h t h e high frequencies of the pollen of Carpinus betulus, a tree with an essentially central E u r o p e a n distribution, indicates a relatively w a r m climate, p r o b a b l y r a t h e r c o n t i n e n t a l .

I n z o n e h—i the m o r e n o r t h e r n character of t h e vegetation is s h o w n b y the fall in frequency of the pollen of Quercus, Carpinus a n d Corylus. H o w e v e r , species of distri­ bution categories 4 , 5 a n d 6 are present in z o n e h—i, in p a r t i c u l a r such southern a n d c o n t i n e n t a l species as Linum anglicum a n d Petroselinum segetum a m o n g s t the l a n d flora, a n d such a c o m p a r a t i v e l y southern species as Potamogeton acutifolius a m o n g s t the a q u a ­ tics. As w i t h the mollusca, it a p p e a r s t h a t certain r a t h e r southern species linger on i n t o the pine-birch zone.

A t t h e top of t h e succession species of groups 5 a n d 6 h a v e v i r t u a l l y disappeared, a n d the d o m i n a n t species h a v e a w i d e r a n g e n o r t h w a r d s into Scandinavia. I t is n o t certain w h e t h e r these u p p e r frequencies h a v e been affected b y t h e secondary deposition a l r e a d y suggested to explain the anomalous behaviour of t h e pollen curves.

X . T h e q u e s t i o n of s e c o n d a r y d e p o s i t i o n T h e curious n a t u r e of the pollen d i a g r a m f r o m t h e t o p of core 3 has been discussed a n d a t t r i b u t e d t o secondary deposition. T h e q u e s t i o n immediately arises w h e t h e r t h e a b r u p t changes in mollusca and significant changes in macroscopic p l a n t remains m a y be due to t h e same cause.

T h e m a i n molluscan change is t h e enormous increase of Bithynia tentaculata, a species possessing a resistant operculum which is liable t o be f o u n d derived from earlier beds. T h e figures quoted in T a b l e I I for this species are counts of opercula, which are a l w a y s m o r e

142

B. W. Sparks Sc R. G. West

frequent t h a n shells because they are less r e a d i l y smashed, b u t the numbers of opercula exceed t h e numbers of shells a t each h o r i z o n usually b y a t h i r d or less, q u i t e a n o r m a l figure a n d n o t one for t h e e x p l a n a t i o n of which a hypothesis of secondary deposition is required. F r o m this it seems t h a t the changes in freshwater mollusca m a y be a reflection of t h e m o r e powerful s t r e a m which was responsible for i n t r o d u c i n g the d e r i v e d pollen.

H o w e v e r , w h e n one looks a t the earlier records of mollusca, some features a p p e a r which m a y well be due to secondary deposition. Helicella crayfordensis has been r e c o r d e d from t h e o v e r l y i n g gravel in 1 9 5 3 (SPARKS in W A L K E R ) . B o t h of these must be in zone from the u p p e r m o s t h o r i z o n , d e p t h 3 metres, in 1 9 5 0 a n d from material r e p u t e d t o come h—i a n d so must the r e c o r d of Corbicula fluminalis ( H O L L I N G W O R T H , ALLISON & G O D W I N 1 9 5 0 ) a t a d e p t h of 3 . 5 - 4 metres. T h e latter is a tough shell which might well s t a n d r e d e position a n d t h e former is certainly more characteristic of t h e m i d d l e of interglacials t h a n t h e ends.

W h e t h e r the shells a r e redeposited or m a n a g e to survive in a climate of increasing severity for a considerable time is i m m a t e r i a l from a geological v i e w p o i n t : the i m p o r t a n t thing is t h a t t h e presence of southern species in the second half of an interglacial does not necessarily m e a n the same as it w o u l d in t h e first half, w h e r e southern species seem to come in q u i t e late (SPARKS 1 9 5 7 ) . I t emphasises the need t o e x a m i n e if possible a succes­ sion of h o r i z o n s to see w h e t h e r a climatic t e n d e n c y can be established before i n t e r p r e t i n g a fauna c o n t a i n i n g isolated southern species. F r o m a biogeographical v i e w p o i n t it is u n f o r t u n a t e t h a t it seems possible to do no m o r e t h a n guess w h a t has been r e d e p o s i t e d and w h a t has n o t .

N o t e s o n c e r t a i n s p e c i e s of m o l l u s c a Bithynia leachi var. inflata. The number recorded probably represents a minimum as most of the shells are broken and the operculum does not always appear to be a safe guide.

Planorbis planorbis. Probably the very broken and juvenile material recorded as Planorbis (Planorbis) sp. belongs to this species, as no trace of P. carinatus has ever been found in these beds.

Planorbis laevis certainly occurs, though very broken and juvenile material is difficult to separate from Planorbis (Planorbis) spp. - hence the doubt expressed at some horizons in Table II. Planorbis leucostoma includes the two forms which might be recorded as spirorbis and leuco­ stoma, the latter being more depressed and having more gradually increasing whorls.

Cochlicopa lubrica. Altough most of the specimes of Cochlicopa are apical fragments, they seemed too inflated for any to be attributed to C. lubricella.

Azeca goodalli. As far as can be seen all specimens should be referred to A. menkeana C. PFEIFFER. The questions of the systematic position of this 'species' and its distribution, recent and fossil, were being considered by Mr. Hugh WATSON before his recent untimely death.

Vertigo moulinsiana. Only the upper half of a shell was seen and this was lost. Hence, it is recorded with doubt.

Arianta and Cepaea. Although the presence of both genera was established from the orna­ mentation of fragments of the outer whorls of the shells, the worn apices of the two genera cannot be separated.

Hygromia liberta is the form with a more raised spire and very narrow umbilicus often regarded as a distinct species and recorded by Kennard in the 1 9 5 0 list.

Agriolimax species are recorded with doubt because of difficulties of recognition discussed elsewhere (SPARKS 1 9 5 7 ) . The forms of the shells attributed to the species are the same as those illustrated in that paper (Plate I, Fig. g and Plate I I , Figs. h-j).

Sphaerium lacustre is based mainly on counts of presumed prodissoconchs. These seem to sepa­ rate cleanly from the rest of the shell and to be very different in detailed character from immature Pisidia, which they resemble in size. A few have parts of the almost straight dorsal margin attached.

R e p o r t o n m o s s e s f r o m t h e i n t e r g l a c i a l d e p o s i t s at H i s t o n Road, C a m b r i d g e

by M. C. F. PROCTOR

(Department of Botany, University of Exeter) The Bryophyte remains examined consist entirely of hypnoid mosses. They include a number of well-preserved pieces of leafy shoots, but a good deal of the material is rather fragmentary and often corroded, so that some of the identifications are necessarily tentative. Nomenclature in

Interglacial deposits at Histon Road

143

the list below follows the check-list by RICHARDS & WALLACE in Volume 1 of the Transactions of the British Bryological Society.

Cratoneuron filicinum. Well preserved leafy shoots in 7 TB and 7 B. Amblystegium kochii. Well preserved leafy shoot in 8 TT. Drepanocladus cf. aduncus. Fragments probably referable to this species in 9 B and 10 TT. Drepanocladus cf. fluitans. 9 TB and the samples from 10 TT downwards contain abundant remains of a Drepanocladus which is almost certainly this species. A little D. exannulatus may be included.

Drepanocladus sp. 7 TT. Acrocladium cordifolium. A short piece of leafy shoot in 9 B. Camptothecium nitens. Common in the higher samples, from 7 B to 8 BTB, usually as short pieces of leafy shoot. cf. Brachytbecium rutabulum. A large but somewhat corroded leafy shoot in 8 TT. cf. Eurhynchium praelongum. 8 BTB and 11 TB. cf. Eurhynchium sp. 8 BTT. The lower samples, with abundant Drepanocladus fluitans, suggest a well-developed mildly acid fen. Those from 8 BTB upwards suggest more calcareous and drier conditions. The samples as a whole, and in particular the higher samples with Camptothecium nitens, suggest a relatively cool temperate climate. C. nitens is much commoner in Scandinavia than in Britain at the present day, though it must be remembered that it is widepread in central Europe, and has widely scattered localities in Britain at the present d a y ; it has been at least locally abundant in Britain during the Post-glacial Period.

References ANDERSEN, S. T., 1957: New investigations of interglacial fresh-water deposits in Jutland. A preli­ minary report. - Eiszeitalter und Gegenwart 8, S. 181-186.

BOYCOTT, A. E., 1934: The habitats of land mollusca in Britain. - J. Ecol. 22, S. 1-38. - - 1936: The habitats of freshwater mollusca in Britain. - J. Anim Ecol. 5 , S. 116-86.

CHRISTY, M., 1924: The hornbeam in Britain. - J. Ecol., 12, S. 39-94.

EHRMANN, P., 1933: Die Tierwelt Mitteleuropas Il(i). Mollusken (Weichtiere). - Leipzig.

ELLIS, A. E., 1940: The mollusca of a Norfolk broad. - J. Conch. 21, S. 224-243. - - 1951: Census of the distribution of British non-marine mollusca. - Ibid., 23, S. 171-244.

FAEGRI, K., & IVERSEN, J., 1950: Textbook of modern pollen analysis. - (Munksgaard) Copen­ hagen.

GEYER, D.. 1927: Unsere Land- und Süßwasser-Mollusken. 3rd. ed. Stuttgart.

HOLLINGWORTH, S. E . , ALLISON, J. & GODWIN, H., 1950: Interglacial deposits from the Histon Road, Cambridge. - Quart. Journ. Geol. Soc. London, 105, S. 495-510.

HUBENDICK, B., 1947: Die Verbreitungsverhältnisse der limnischen Gastropoden in Südschweden. Zool. Bidr. Uppsala, 24, S. 421-559.

HULTEN, E., 1950: Atlas of the distribution of vascular plants in N.W. Europe. - Generalstabens Litografiska Anstalts Förlag, Stockholm.

JESSEN, K. & MILTHERS, V., 1928: Stratigraphical and palaeontological studies of interglacial fresh-water deposits in Jutland and northwest Germany. - Danm, Geol. Unders., II Raekke, Nr. 48.

SPARKS, B. W., 1953: Fossil and recent English species of Vallonia. - Proc. Malac. Soc. London, 30, S. 110-121. - - 1957: The non-marine mollusca of the interglacial deposits at Bobbit­ shole, Ipswich. - Phil. Trans. Roy. Soc. London, B, 241, S. 33-44.

THOMSON, P. W., 1951: Das Interglazial von Wallensen im Hils. - Eiszeitalter und Gegenwart, 1, S. 96-102.

VON DER BRELIE, G., 1954: Transgression und Moorbildung im letzten Interglazial. - Mitt. geol.

Staats-Inst. Hamburg, 23, S. 111-118.

WALKER, D., 1953: The interglacial deposits at Histon Road, Cambridge. - Quart. Journ. Geol.

Soc. London, 108, S. 273-282.

WEST, R. G., 1956: The Quaternary deposits at Hoxne, Suffolk. - Phil. Trans. Roy. Soc. London, B, 239, S. 265-356. - - 1957: The interglacial deposits at Bobbitshhole, Ipswich. - Ibid., 241, S. 1-32.

Manuskr. eingeg. 4. 2. 1959.

Anschrift der Verf.: Mr. B. W. Sparks, Department of Geography, University of Cambridge.

Cambridge/England. — Dr. R. G. West, Department of Botany, University of Cam­ bridge, Cambridge/England.