THE SEDIMENTOLOGY OF A LATE PLEISTOCENE DRUMLIN NEAR KINGSCOURT, IRELAND

An exposure in a Late Pleistocene drumlin near Kingscourt, Ireland, pr ovides a good insight into some of the processes that give rise to suc h subglacial bedforms. The drumlin is located on a ridge, cored by red sandstone of Carboniferous (Namurian) age, which rises to 150 m above m.s.l. The drumlin itself is about 380 m long by 170 m wide and is 20 m in height. It is orientated WNW-ESE. Ice flow direction in the area , as inferred from general drumlin orientation, striae, and erratic di spersal, was NW-SE. The drumlin is composed of diamicton (with four co nstituent facies), containing large-scale (up to 20 m long and 1.5 m h igh) slabs of the sandstone bedrock, which have been displaced tens of metres by ice dragging. The thin diamicton matrix is sand-rich with f ew clasts greater than pebble size. It contains green sandstone errati cs from west and northwest of the study site along with clasts of weat hered Namurian sandstone which have been sheared from local bedrock. T he diamicton attains a maximum thickness of 5.4 m. The slabs are confi ned to the basal 2.5 m of the diamicton. Thus the drumlin is essential ly rock cored.Parts of the matrix are interpreted as injection sedimen ts that have been squeezed into fractures and voids in the bedrock, an d between rock slabs, under high porewater pressures, when the ice beg an to displace fractured substrate. Deformation structures at a variet y of scales are seen in the matrix. Deformation, erosion and depositio n were all important in the formation of the drumlin. Much of the uppe r part of the diamicton has been sheared, subsequent to initial deposi tion, by ice action. The sheared units are uppermost in the stratigrap hic sequence. The shearing is the last glacial process apparent in the drumlin sediment and may have been contemporaneous with drumlinisatio n. Both the squeezing process, which deposited the injections within t he matrix, and the deflection in flow of ice were influenced by the ob structing bedrock ridge which, in this case, is also responsible for t he anomalous orientation of the feature.