A new approach to the relief of Great Britain: II. A classification of rocks based on relative resistance to denudation

We have assembled a digital database for Great Britain, containing square by square data for the 1 km squares of the National Grid on altitude and geology (age and lithology) as well as other variables. It is used here to classify the 71 most common rocks of Great Britain (i.e., all outcrops over 500 km2) in terms of the degree to which they form high ground. The empirical classification utilised a large number of measures (eventually 19) to allow for the varying location and spatial pattern (coastal vs. inland, large continuous outcrops vs. scattered patches) of these outcrops, and several different combinations of variables produced very similar sequences when the rocks were arranged in rank order. A six-fold classification provides a basis for further work which will seek to remove the influence of varying resistance to denudation from the pattern of relief. For rocks of similar age, rank order is determined by lithology, but for all rocks, age is a stronger influence than lithology. It is shown that variations in the resistance of rocks to erosion account for a large part of the local relative relief of Britain. The pattern of mean altitude predicted from river distance values using regressions for each rock resistance class simulates remarkably closely the overall pattern of British relief.