The Coastal Periglacial Slope Deposits of the Cotentin Peninsula

The head deposits of Cotentin form a terrace sloping seaward and are generally interpreted as being banked against an old cliff line. They are frequently seen to rest on raised beach gravels at the junction of the head and this rock cliff. In the central parts of the bays, the basal beds up to 2 m thick consist of grey silty and gravelly flows sometimes enclosing lenses of organic silt. These beds appear to rest on bedrock and have been found in hollows in the present shore platform. Overlying them there may be about 2 m of silt, sand and gravel, some beds being openwork, especially on granite, arkose and quartzites; they appear to be weakly developed on greywackes and shales. Above these there is a coarse angular and roughly stratified head which makes up the greater part of the exposure. These deposits show many features similar to those of the superficial deposits at Morfa-bychan, south of Aberystwyth in west Wales. Fabric studies at sixteen sites show that the preferred stone orientation patterns are comparable, while the roundness index calculated on the Cailleux formula lies in a similar range. The thickness exposed in the cliffs at both Morfa-bychan and Herquemoulin reaches 30-40 m below slopes of 160-170 m on the same rock type. THIS STUDY resulted from research on the superficial deposits exposed in the cliffs at Morfabychan, south of Aberystwyth in Wales. We concluded that these deposits are periglacial slope deposits derived almost entirely from the Silurian greywacke behind (Watson, 1967), though they had previously been interpreted as glacial drifts. Apart from the absence of erratics (with the exception of a very minor fragmentary deposit containing Irish Sea erratics, all the material could have come from the rock slope above), there are four main points in favour of a periglacial origin: (i) the deposits thicken as the height of the backslope increases; (2) the bedding dips regularly seawards; (3) the preferred stone orientation is consistently sub-parallel to the slope; and (4) the dips of the long axes of the stones approximate closely to the angle of dip of the beds. As a check on the periglacial hypothesis, we decided to examine a comparable area of coastal periglacial slope deposits accepted as lying outside the limits of the Pleistocene ice. Some of the north-western parts of the Cotentin Peninsula in Normandy are suitable for such a comparison. South-west of Beaumont Hague, the backslope is of a similar height, rising to just over I80 m, which is the maximum elevation in the Morfa-bychan area, and the rocks are of a similar type, consisting mainly of Cambrian and Ordovician greywackes, mudstones and shales. Work was concentrated mainly on the fine cliff exposures of the Bay of Ecalgrain and the north end of the Bay of Vauville around Herquemoulin, though data were collected between Landemer and Rozel, and the coastal sections were examined east of Cherbourg as far as Reville (Fig. I)

Archaeology · Geography · Geomorphology · Peninsula · Physical geography · Coastal and Marine Dynamics · Geological formations and processes · Geology · Geology and Paleoclimatology Research