The Geology of the Round towers of County Down by Ian Meighan

The Geology of the Round Towers of County Down

By Ian Meighan

Irish Round towers still retain an element of mystery regarding their exact number and functions. Geoarchaeologically, however, one can generalise that they tend to have been constructed from locally available (including glacially-transported) rock materials. table 1 provides geological information for the Round towers at nendrum, Drumbo and Maghera.

Interestingly, Nendrum and Drumbo (respectively NE and N of Slieve Croob, Map 2) lack granite outcrops in their immediate vicinity and do not contain this lithology. Both are on Silurian Gala Group bedrock. By contrast, Maghera, situated on (unexposed) Silurian Hawick group bedrock between the Newry and E Mourne granitic bodies (Map 2), has quite abundant Newry granodiorite material. This is in a water-rounded form and undoubtedly reflects initially angular fragments transported by Pleistocene ice which moved from NE to SW in this region. However, Mourne granite fragments would not have been carried northwards by ice to the Maghera area: it is also significant that the spectacular Slidderyford Neolithic Dolmen (portal tomb), just over 2 km E of Maghera, involves only glacially transported Newry granodiorite (3 large fragments) as well as Silurian rock, but no Mourne granite material.

The lithology of these monuments reflects their local Geology, e.g. the proliferation of Lower Palaeozoic (probably Silurian) greywacke sandstones/siltstones. The absence of granite at Nendrum and Drumbo is also noteworthy in this context. The abundance of water-rounded Newry granodiorite boulders at Maghera undoubtedly indicates material which was initially transported by Pleistocene ice as angular fragments derived from the nearby Newry Igneous Complex.

Round Tower Principal rocks subsidiary rocks absent rocks nature of rock fragments

nendrum

Lower Palaeozoic greywacke sandstone (+siltstone) Red sandstone (PermoTriassic) Granite, basalt/ dolerite

Some waterrounded material

Drumbo Lower Palaeozoic greywacke sandstone

Siltstone (Lower Palaeozoic) quartz-rich sandstone (Carboniferous ?), vein quartz Granite, dolerite Very little waterrounded material

Maghera Lower Palaeozoic siltstone (+greywacke/ slate) Newry granodiorite (quite abundant), Mourne granite (G2) (rare), microgranites, basalt Dolerite Water-rounded material present (e.g. Newry granodiorites)

Downpatrick No original material survives in situ

Table 3 The Geoarchaeology of County Down Round Towers