THE GLACIAL PERIOD. 183 



and rivers. Again, it is evident that in great valleys like the 

 basin of the Forth, and wide spaces like Strathmore, and the 

 low-lying districts generally, the ice would have a sluggish 

 motion, and would, in such places, tend to accumulate sub- 

 glacial debris to a much greater extent than in regions where the 

 slope of the ground was considerably greater. All these expect- 

 ations we find fully realised throughout the length and breadth 

 of Scotland, — the till is distributed exactly as it ought to be, 

 upon the supposition that it marks the bottom-moraine of an 

 old mer de glace. And, what is still more suggestive of its 

 origin, it is frequently arranged in the form of long broad 

 smoothly-outlined ridges or "drums" and "sow-backs," as they 

 are called, the trend of which exactly coincides with the direc- 

 tion of the striae upon the underlying rocky pavement. These 

 drums are especially conspicuous in the lower reaches of the 

 Tweed in Eoxburghshire and Berwickshire, and are well brought 

 out upon the shaded one-inch map of the Ordnance Survey. 

 The Drums of Nithsdale are also a fine example of the same 

 phenomena. ISTo one, however ignorant of glacial geology, can 

 look at those maps without feeling convinced that the whole 

 region has been acted upon by some great agent moving in one 

 and the same determinate direction. In Teviotdale and Tweed- 

 dale all the ridges, whether of boulder-clay or solid rock, are 

 seen sweeping down the main valley in exactly parallel lines. 

 Here and there are prominent hills shooting abruptly upwards, 

 each showing a steep face towards the region whence the abrad- 

 ing force moved, but sending out a long and narrow sloping 

 bank of detritus behind. The drums bore the same relation to 

 the old ice-sheet that the long ridges of gravel and sand in the 

 bed of a river do to the current that heaps them up and is con- 

 tinually modifying them. 



That boulder-clay consists of the debris of the rocks is 

 sufficiently evident. The stones are the more or less worn and 

 abraded fragments which have been detached during the grind- 

 ing of the ice and the slow rolling -forward of its bottom- 

 moraine. Many of these fragments have been carried a long 



