Ice-marginal and sub-marginal landforms that formed during the recession of former ice sheets are widely used to reconstruct ice retreat dynamics, including shifting thermal conditions and marginal oscillations. This information is used to predict the future behaviour of modern ice sheets during recession. The last Irish Ice Sheet is of interest in this context, due to its southerly position and proximity to the North Atlantic. Prominent moraines along the coast and on the continental shelf clearly indicate ice margin positions during retreat. However, there are few well-developed moraines inland. Instead, hummocky topography, interpreted as evidence of ice stagnation, has been used as evidence for former ice-marginal positions. However, hummocky topography may form in a variety of ways and in a number of positions in relation to the ice margin.
In the central Irish Midlands hummocky topography has been previously used to indicate the position of the Drumlin Readvance Moraine (DRM), originally proposed to have formed during a readvance associated with the formation of extensive drumlin fields in the northern half of the island. However, the position and nature of this inland ice margin, and the evidence for ice readvance, remains under debate. Here we use a combination of LiDAR, mapping from high-resolution air photos, ground-penetrating radar (GPR) surveys, and sedimentological analyses to examine hummocky topography in 3D in the central Irish Midlands, in order to establish (i) how the topography formed and (ii) inform ice sheet dynamics during deglaciation.
Results show that extensive areas previously mapped as hummocky topography actually consist of multiple short, ice-marginal, diamicton ridges with streamlined crests, that have been superimposed on the remnants of mega-scale glacial lineations. We interpret these features as evidence for accelerated ice flow and ice readvance. A second type of hummocky topography, consisting of geometric arrangements of mounds, with evidence of normal faulting along the margins of individual mounds indicates supraglacial deposition of sediments within crevasses during ice melting. We conclude that at least one significant readvance of the Irish Ice Sheet associated with accelerated ice flow occurred in the central Irish Midlands, but that this post-dated drumlin formation.