RECONNAISSANCE GROUND-PENETRATING RADAR STUDIES OF RAISED SHORELINES IN DONEGAL, REPUBLIC OF IRELAND

In June, 2013, we conducted Ground-Penetrating Radar (GPR) studies across two raised shorelines in Donegal: 1) Malin Head and 2) Greencastle. We collected about 3 km of GPR data with Pulse EKKO 1000 200 MHz and 50 MHZ antennae and obtained 3-d positioning from a TRK-GPS. At Malin Head, two shorelines, one at 25 m and the other at 4 m above sea level, are cut into glacigenic deposits and a 3 m high gravel beach shelters the raised features. The glacial-age material is in some outcrops a muddy, matrix-supported conglomerate, though discrete sand and gravel layers are apparent in many other places, along with till in the highest areas. The upper shoreline complex is largely till covering a bedrock platform with thin and patchy pockets of former coastal sand and gravel deposits in a sloping terrace of largely eroded till. On the lower terrace, GPR observations consistently show steeply inclined reflectors dipping seawards for more than 100 m from the raised shoreline terraces. It appears that the lower raised shoreline is composed of a formerly eroded Pleistocene/Holocene source for sand and cobbles with a regressive series of steeply inclined former foreshore deposits. At Greencastle, on Lough Foyle, we collected a 1.5 km grid of 200 MHz GPR lines on a 2-4 m high, seaward dipping surface above a modern, low-energy sand and gravel beach. Exposures in a bluff behind the modern beach revealed discontinuous layers of poorly sorted sand and gravel analogous to the modern beach. The GPR observations revealed similar, discontinuous reflectors beneath the plain extending several hundred meters landward to a paleo-bluff of glacial material. In this instance, it appears that the 2-4 m shoreline was derived from erosion of the older glacial sediment just as the modern beach is sourced by the late Holocene, raised beach. In each of the two locales, no constructional landforms remain from the raised shoerelines, and the thickness of raised Holocene deposits increases away from the Pleistocene bluff.