PEBBLES
THE IMPLICATIONS OF “WEAR AND TEAR” ON FLINT PEBBLES IN SOME LOCAL PLEISTOCENE DEPOSITS HOWARD MOTTRAM
Introduction
We can find pebbles in Suffolk that are composed of quartz, quartzite, chalk, chert and a few other rock types but the vast majority are composed of flint. In the Norwich Crag, notably in its Westleton Beds, it is in fact difficult to find pebbles that aren’t composed of flint. It has often been said that the flint pebbles in the Norwich Crag are well rounded, a characteristic that has been used to indicate that the pebbles have been worn by lengthy and/or strong periods of attrition; but, is this quite right?
The creation of flint pebbles
Flints in the Chalk would have been of variable size and shape but, invariably, they would have had curved surfaces. When the Chalk was subjected to weathering and erosion, the brunt of the attacks would have been taken up by the soft chalk matrix so releasing individual flints. Although flints are very strong, they are brittle and narrow cylindrical (rod shaped) types and those with projecting “horns” or “knobs” would have been most vulnerable to damage. Initial damage by processes such as shattering by freeze-thaw, collision with one another when falling out of a Chalk exposure, collisions in turbulent water (e.g. the highland headwaters of a river, meltwater under pressure), or shearing at the base of an ice sheet, would have resulted in fracturing that produced smaller flints with sharp edges, i.e. pebbles with angularity.
In rivers
If the now angular flint pebbles were deposited soon after initial damage, then they would have remained like this, largely angular.
Should the flints have stayed in river systems and been rolled and bounced downstream on the bed of large rivers for sustained lengths of time, then subsequent damage would have chipped away at the angular edges of the flints and further wear would have been inflicted by the slow process of abrasion. Abrasion would have been caused by rubbing against bedrock, sand and other pebbles and it would have affected the entire surface of moveable pebbles so reducing their size. It has long been held that abrasion not only makes pebbles smaller but that it can also transform angular pebbles into rounded ones (see for example, Domokos et al., 2014). There are several factors involved in the effectiveness of abrasion so that this pair of outcomes, size reduction and rounding, is not always evident (Sherman et al., 2013, p.243; Bridgland, pers. comm.). The flints in the Kesgrave Sands & Gravels were obtained from the Chalk of the Chiltern and North Downs areas and were, therefore, transported long distances but often over relatively soft and poorly abrasive bedrocks such as the Chalk itself and the London Clay. The flints in the Kesgrave Sands & Gravels do show an increase in rounding, albeit that it is more strictly a slight reduction in angularity (see also Bridgland, 1999, p.108). Conversely, the flints in the Ingham Sands & Gravels were obtained from the Chalk in the Ingham area but were soon deposited so that they had little opportunity of being rolled into rounder pebbles (Bridgland, 1999, p.109) and they remained angular, as indicated in Table 1.
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TABLE 1
ROUNDNESS OF FLINT PEBBLES IN SOME PLEISTOCENE RIVER SEDIMENTS
(all pebbles in the 32-16mm size range)
STRATUM DEPOSITED BY ANGULAR TYPES % ROUNDED TYPES %
Haughley Park Gravels (often very chalky)
aggressive river (pulses of torrential flow ?) draining in front of an ice-sheet; flints soon deposited so minimal rounding
Sandy Lane Gravels (sometimes chalky) streams/river draining in front of an ice-sheet; flints soon deposited so minimal rounding
Barham Sands & Gravels (rarely chalky) streams/river draining in front of an ice-sheet; cuts across and probably reworked flints from the Kesgrave Sands & Gravels as flint roundness similar
Kesgrave Sands & Gravels (not chalky) large river (proto-R. Thames) in a temperate climate; obtained flints from distant chalklands, so lengthy journey; consequently, flints more rounded
Ingham Sands & Gravels (occasionally chalky) large river in a temperate climate; obtained flints from local Chalk but flints very soon deposited; consequently, negligible rounding of flints
90 10
91 9
79 21
75 25
100 0
Data is for total flint content – i.e. Includes flints determined as recycled from Paleogene (Tertiary) Beds. Analyses mostly taken from Allen, 1984a, p.16 and Allen, 1984b, p.34. The Ingham Sands & Gravels data is from Bridgland, 1999, p.104.
In the sea
The flint pebbles in the Norwich Crag have usually been well sorted by the sea so that only a fairly limited size range is normally displayed in exposures. As a repercussion, we are inclined to have a restricted appreciation of pebble roundness over a range of sizes. Nonetheless, even the limited amount of published data that I could find, shown in Table 2, suggests that these marine pebbles do not become more rounded as they become smaller, instead they become more angular.
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TABLE 2
ROUNDNESS OF FLINT PEBBLES IN SOME NORWICH CRAG GRAVELS
ANGULAR TYPES % ROUNDED TYPES % SIZE RANGE mm
REFERENCE 44 57 32-16 Hey, 1967, p.430 (see also p.432). 18 82 Bridgland, 1999, p.104. 64 36 16-8 Clarke & Auton, 1984, p.110. 51 49 Hey and Auton, 1988, p.36.
The data taken from the quoted references has not been filtered (some samples that the authors used may now be considered younger than the Norwich Crag)
This is in contrast to the stereotypical trend of river rolled pebbles mentioned earlier, but it is in agreement with what was reported for the marine deposited flint, and chert, pebbles on Chesil Beach (Carr, 1974, p.866) and also for the Norwich Crag at Wangford (Clarke & Auton, 1984, p.109). Further analyses of Norwich Crag pebbles, including analyses of some less well sorted samples, are shown in Table 3. Here, each sample was analysed for a few different size ranges. Despite the vagaries of individual samples and the fact that usually only 3 size ranges were investigated, the data does support the notion that these marine deposited pebbles had become less rounded (more angular) as they became smaller.
In the nearshore area of the sea during Norwich Crag times, flint pebbles could have accumulated in locations such as the feet of any cliffs, on beach foreshores, in tidal channels and in river mouth shoals. In particular, when there was an abundance of pebbles, the pebbles would have formed ridges a few metres thick on the higher levels of the beaches. On the seaward facing surfaces of those ridges, at around the times of the twice daily high tides, the run-up of swash from normally gentle to moderate energy waves would have slid and rolled the gravel, and perhaps cobble, sized flint pebbles against each other. Under these sea conditions, longshore currents would have also been restricted to sliding and rolling similar sized material. Flints usually have an isotropic fabric so that any shaping caused by abrasion mechanisms, over hundreds to thousands of years, would not have been influenced by the fabric of the flints but by the equality or inequality of the abrasion. If the flints were not equant such that they were more easily slid than rolled, then the rounding caused by abrasion would have been uneven and would have resulted in flatter (disc and blade shaped) pebbles; if the flints were equant and rolling was the dominant mechanism, then the rounding would have been even and would have resulted in spherically shaped pebbles. The wearing away of the pebbles not only caused rounding but it also resulted in the gradual size reduction of the pebbles. Abrasion would have been intermittently interrupted when rough to stormy seas disturbed a greater depth and width of the ridges of flint pebbles, threw the flints about and crashed the flints against one another so that even virtually unmovable
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TABLE 3
ROUNDNESS RELATED TO SIZE FOR FLINT PEBBLES IN THE NORWICH CRAG
LOCATION SIZE mm ROUNDED TYPES % ANGULAR TYPES % VERY ROUNDED MODERATELY ROUNDED SUB ROUNDED SUB ANGULAR MODERATELY ANGULAR VERY ANGULAR
Easton Bavents Cliff ‡ rip channel (gravel fraction moderately sorted)
North’n Covehithe Cliff * rip channel (gravel fraction well sorted)
Central Covehithe Cliff * +7.5m OD (gravel fraction moderately sorted)
Central Covehithe Cliff * +6.5m OD (gravel fraction poorly sorted)
Central Covehithe Cliff * +5.5m OD (gravel fraction moderately sorted)
Wangford † gravelly sand of subtidal channel in tidal flats (gravel fraction poorly sorted)
>32 0 100 0 0 0 0 32-20 54 39 7 0 0 0 20-16 28 81 17 3 0 0
>32 100 0 0 0 0 0 32-20 43 32 25 0 0 0 20-16 28 56 16 0 0 0
>32 0 13 28 22 37 0 32-20 43 21 7 12 15 0 20-16 0 58 30 11 0 0
>32 93 4 0 0 3 0 32-20 78 18 4 <1 0 0 20-16 32 26 33 7 2 0
>32 100 0 0 0 0 0 32-20 43 32 25 0 0 0 20-16 28 56 16 0 0 0
>32 0 0 0 0 0 0 32-20 100 0 0 0 0 20-16 0 100 0 0 0 0 16-10 0 100 0 0 0 10-6 0 0 0 93 7
‡ Cliff significantly receded since sampled but other rip channel sediment available here. * Cliff significantly receded since sampled. Detailed location and levels in Mottram, 1989. † Pit infilled since sampled. Location in Mottram, 1988.
boulder sized flints would have been subjected to collisions. Repeated collisions of the flints, especially spherically shaped specimens, would have produced the tiny crescent shaped chips in their surfaces that are known as “chatter marks” (percussion scars). Occasional dramatic collisions would have caused brittle fracture that resulted in the breakage of large flints into smaller and more angular pieces. These smaller and more angular flints would have been prone to further breakage as smaller (lighter weight) pieces would have been more readily thrown about and both smallness (less mass) and angularity would have made them more susceptible to the stresses imposed by collisions. When the normal tide and wave conditions resumed, the pebbles at the surface would once again have been slid and rolled about. As ridges of pebbles usually
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lack sand in the spaces between the pebbles, some small flint pebbles would have been able to be shuffled beneath the surface layer of large pebbles, so affording protection of the small pebbles from abrasion and rounding. Therefore, continued diminution of the size of the flints in the sea under rough to stormy conditions during Norwich Crag times, created angular rather than rounded pebbles.
Conclusions
Flints in the Chalk had curved surfaces. As flints were released from the Chalk some damage may have occurred then but most early damage would have occurred after they were captured by rivers or ice. Breakage of the flints under these conditions introduced angular edges.
In the river systems that developed from glacial melt waters, the flints were obtained from nearby Chalk and the flints were buried relatively soon afterwards so there was minimal opportunity for the angled edges to become rounder. Likewise, the river that deposited the Ingham Sands & Gravels (Bytham or Ingham River) cut through local chalklands where it obtained flints which were soon deposited so that they too had little opportunity of being rolled into rounder pebbles. On the other hand, where flints were obtained from distant chalklands, the pebbles were rolled for a long time on the bed of large rivers, such as the proto-River Thames (manifested as the Kesgrave Sands & Gravels). In this case, further damage (chipping away) of the edges initiated rounding which was enhanced by abrasion such that the pebbles got smaller as they became less angular (more rounded).
It is by no means certain, but it has been thought most likely that a river, possibly a forerunner of the Bytham or Ingham River, delivered flints into the area during the time when the Norwich Crag was being deposited. Either, these flints were unloaded directly into the North Sea from where they were immediately available to marine processes, or these flints were laid down in river terraces from where the sea subsequently obtained them when it transgressed westwards. If the flints were not acquired by the sea in the proximity of Southwold then they must have been conveyed here via southward directed longshore drift. Both longshore drift and the uprush of waves near high tide caused abrasion that resulted in rounding of large flint pebbles. This rounding was much greater than that of any of Suffolk’s Pleistocene river pebbles as abrasion in the sea was more vigorous and, or, over longer time spans. The effects of periodically rough and stormy wave conditions were dramatic as these conditions threw the flints around and smashed them into one another. With this continued but aggressive “wear and tear”, the marine pebbles did not become more rounded but more angular as they got smaller. Therefore, although marine pebbles that are rounded may indicate that they have been subjected to distinct abrasion, we need to check the roundness of other sizes and assess the significance. Taking the Norwich Crag as an example, if analyses of pebbles show that the pebbles become less rounded (more angular) as they become smaller, then this indicates that the pebbles were not just subjected to abrasion but that they also experienced aggressive attrition.
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Notes
• Pebbles is a non-specific term that is widely used to cover cobbles and gravel. Where the latter terms for size have been used in this text, they are as defined in ISO 14688-1:2017, whereby;boulder >200mm dia., cobble 200-63mm dia., gravel 63-2mm dia., sand 20.063mm dia.
• The angularity v roundness of my pebbles was determined by comparison against published roundness charts (diagrams showing pebble outlines/ silhouettes). It is likely that this was also the method used for the data obtained by the other authors quoted.
• The natural shapes of flints and features such as “chatter marks” are well illustrated on the GeoSuffolk website, at https://geosuffolk.co.uk/images/ Leaflets/flintleaflet.pdf
References
Allen, P. (1984a). Kesgrave sands and Gravel Formation. in Allen, P. (ed). pp.1524.
Allen, P. (1984b). Lowestoft Formation. in Allen, P. (ed). pp.32-47.
Allen, P. (ed), (1984). Field Guide (revised edition, October 1984) to the Gipping and Waveney Valleys, Suffolk. Quaternary Research Association, Cambridge. Bridgland, D. R. (1999). Analysis of the raised beach gravel at Boxgrove and related sites. in Roberts, M. B. & Parfitt, S. A. pp.100-111.
Carr, A. P. (1974). Differential movement of sediment particles. Conf. Coastal Eng., 14th, Copenhagen, Am. Soc. Civil Engineers, Proc. 2: 851-870. (https:// icce-ojs-tamu.tdl.org/icce/index.php/icce/article/view/2944).
Clarke, M. R. C. & Auton, C. A. (1984). The Early Pleistocene of the lower Waveney Valley. in Allen, P. (ed). pp.108-110. Domokos, G., Jerolmack D. J., Sipos, A. A´. & Török A´. (2014). How river rocks round: resolving the shape-size paradox. PLoS ONE 9(2): e88657. doi:10.1371/ Gibbard, P. L. & Zalasiewicz, J. A. (eds), (1988). Plio-Middle Pleistocene of East Anglia. Field Guide. Quaternary Research Association, Cambridge. Hey, R.W., (1967). The Westleton Beds reconsidered. Proceedings of the Geologists’ Association, 78: 427–445. doi:http://dx.doi.org/10.1016/s00167878(67)80008- 7.
Hey, R. W. & Auton, C. A. (1988). Compositions of pebble-beds in the Neogene and pre-Anglian Pleistocene of East Anglia. in Gibbard, P. L. & Zalasiewicz, J. A. (eds). pp.35-41.
Mottram, H. B. (1988). Norwich Crag at Hill Farm Pits, Wangford. Trans. Suffolk Nat. Soc., 24: 88-94.
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PEBBLES
Mottram, H. B. (1989). The Upper Caenozoic sequence around The Long Row, Covehithe. Trans. Suffolk Nat. Soc., 25: 86-91.
Roberts, M. B. & Parfitt, S. A. (1999). Boxgrove. A Middle Pleistocene hominid site at Earham quarry, Boxgrove, West Sussex. English Heritage Archaeological Report 17.
Sherman, D. J., Davis L., & Namikas S. L. (2013). Sediments and sediment transport. in Shroder, J. F. (ed). 1: pp.233-256. Shroder, J. F. (2013). (ed). Treatise on Geomorphology. Academic Press, San Diego.
H B Mottram salhow@talktalk.net
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