Whether a significant thickness of Mesozoic cover rocks once rested upon the Palaeozoic rocks, which form the structural highs of the English Lake District and the Northern Pennines, is a subject of long debate which has been revived recently by apatite fission track analysis (AFTA) studies. Evidence from adjacent basins suggests that 1200–1750 m of Mesozoic cover was probably formerly present on the highs, prior to Cenozoic erosion. Higher estimates (c. 3000 m) of cover thickness, based on such fission track-derived palaeotemperatures, result from underestimation of early Cenozoic surface temperatures and from not allowing for the relatively low thermal conductivity of the eroded cover rocks. For similar reasons, apatite fission track estimates of eroded cover rocks elsewhere in Britain may also prove to be overestimates.

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