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Trimlines mark the boundary between glacially eroded landscapes on low ground and landscapes dominated by evidence of periglacial weathering on higher summits. For many years the trimlines of Scandinavia, Britain and Ireland have been interpreted as marking the surface of the ice sheets at the maximum of the last glaciation, but recent cosmogenic exposure dating of erratics far above the trimlines in NW Scotland shows this to be false. The trimlines in that area must represent an englacial thermal boundary between warm (eroding) ice and cold (protecting) ice. It is now clear that even very experienced geomorphologists cannot necessarily tell the difference between terrain that has been recently glaciated and terrain that has not, because cold-based ice can leave virtually no trace. This calls into question not only the interpretation of high-level trimlines elsewhere, but also the mapping of the lateral limits of past glaciations, which are often based on similar or even weaker geomorphological and sedimentological evidence. [ABSTRACT FROM AUTHOR]