400 



H. J. Lowe — Sketch of a hit of Dartmoor 



ordinary flow is almost hidden several feet below the highest 

 ' steps.' Now it is well known that granite weathers irregularly. 

 Frequently harder cores occur which resist decay and stand above 

 the disintegrated surface. These, by processes due to temperature, 

 denudation, and gravity, gradually make their way to the bottom 

 of the valley. They are to be met with in most granite areas, 

 scattered about in what might paradoxically be called an irregularity 

 that has a kind of uniformity about it. Now Horsham Steps has 

 no parallel that I am aware of, and it seems to me probable that 

 the boulders found their way into the bed of the minor stream in 



Fig. 2. — Former drainage of same area as represented in Fig. 1. 



the usual scattered fashion, but that the somewhat sudden increase 

 of the stream by the addition of the waters of another river with 

 much greater volume increased its force and carrying power 

 sufficiently to drive together the boulders strewn along a rapid 

 portion of its course, in such a manner that they formed a support 

 to each other and thus became an immovable barrier and unusual 

 feature. 



As there are data upon which to base calculations, it will be of 

 interest to form some idea of the time which has elapsed since the 

 river changed its course. Lord Avebury, in his " Scenery of England," 

 cites Croll as estimating the mean rate of erosion by rivers as one 



