1 80 PREHISTORIC E UR OPE. 



about the most important member of the glacial deposits — the 

 Till or Boulder-day. 



The grinding of this enormous mass of ice, exceeding 

 3000 feet in its deeper parts, resulted in the general smoothing 

 away of asperities and sharply-projecting rocks. And the result 

 is seen now in the flowing contour which distinguishes all the 

 hill -ranges of Central Scotland, the greater portion of the 

 Southern Uplands, and all but the loftier peaks and ridges of 

 the Highlands and mountainous islands of Arran and the Inner 

 and Outer Hebrides. As a consequence of all this erosion and 

 abrasion, immense quantities of stony dSbris gathered underneath 

 the ice, and were slowly dragged and rolled forward. In the 

 glacier -valleys of the Alps and Norway similar debris forms 

 below the ice, but since a good deal of water circulates between 

 that ice and the rocky pavement over which it flows, the finer 

 sediment — the sand and mud — is washed out and carried away 

 by the discoloured rivers that leap out at the terminal fronts of 

 the glaciers. That similar streams and torrents and rivers flowed 

 underneath the old mer de glace of Scotland admits of no doubt, 

 for their water-worn gravel and shingle are here and there con- 

 spicuous enough in the heart of the glacial deposits. There 

 appear even to have been what we might call sub-glacial lakelets 

 — hollows underneath the ice in which fine clay slowly accumu- 

 lated, and into which now and then stones were dropped from 

 the over-arching roof of ice. But such streams and lakes pro- 

 bably bore a smaller proportion to the area covered by the ice 

 than the rivers and lochs of the present land-surface do to the 

 dry ground over which they are distributed. And consequently 

 the coarse de'bris and clay and sand were allowed in most 

 places to accumulate undisturbed by the modifying action of 

 water. The stones which were in this way forced along under- 

 neath the ice came in time to have their angles rubbed off, and 

 their faces smoothed, striated, and polished. The accompany- 

 ing illustration (Fig. 5), which is drawn from nature by my friend 

 Mr. B. E". Peach, represents a typical boulder-clay stone. It will 

 be observed that the stone is smooth, and scratched principally 



