Maguiresbridge Under Permafrost
Derryvree matters because its structure records change. Till is debris deposited directly by glacier ice: a dense, unsorted mixture of clay, stones and larger fragments. At Derryvree, a lower till was followed by freshwater and organic sediments, then by another till. The simplest reading is that glacier ice had occupied the locality, withdrew or ceased to cover it for a time, and was later replaced by renewed ice. This is not evidence for an entirely ice-free Ireland. It is evidence that at least parts of the northern landscape could, during the wider glacial cycle, contain exposed ground, standing or slow-moving water and living ecosystems between episodes of ice advance.
