How thick are drumlins; An analysis of drumlin relief for a large sample

To be credible, any theory of drumlin genesis must do a good job explaining the physical shape and dimensions of these curious, but widespread, glacial landforms. It is thus essential that such data is collected. A critical aspect is the ’height’, ’thickness’, or more precisely the relief of a drumlin (i.e. range in elevation from its base to its summit). The literature widely reports drumlin relief to lie in a range between 1 m and 200 m (e.g. Hattestrand et al. 1999), but in most cases we find actual measurements too sparse in number to derive sensible estimates of mean and modal relief and the frequency distribution. In this paper we analyse drumlins from our recent mapping program of Great Britain. The mapping was based on the NextMap DTM (5 m horizontal resolution, 0.7-1 m vertical accuracy) and allowed us to create a GIS database of 26,180 drumlins recorded as polygons, defined their perimeter break-of-slope. If drumlins only lay on flat surfaces, their relief would simply be the highest elevation they reach minus their lowest elevation, and within the GIS we could subtract the minimum elevation of the perimeter from the summit elevation. However, for Britain, many drumlins lie on hillslopes forcing us to develop a new procedure for deriving drumlin relief. Conceptually, the method slices the drumlins from their parent landscape and forces them all down to a common base level. We end up with a DTM of drumlins for the whole of (mainland) Britain as if they were on a flat landscape at zero elevation. From this we derive measures of drumlin relief, and in this paper we will report on these statistics and the patterns of change in relief. Our statistics reveal an important surprise; drumlins in Britain are of much lower relief than was expected.