IV THE ICE AGE AND ITS WORK 65 



a distance to sheep lying down, are perhaps the most 

 general of all the indications of glacial action. Every 

 glacier carries with it, embedded in its under surface, 

 numbers of rocks and stones, which, during the slow but 

 unceasing motion over its bed, crush and grind down all 

 rocky projections, producing in the end gently rounded 

 or almost flat surfaces even on the hardest ancl toughest 

 rocks. In many of the valleys of Wales, the Lake District, 

 and Scotland every exposed rock has acquired this char- 

 acteristic outline, and the same feature can be traced on 

 all the rocky slopes, and often on the summits of the lesser 

 heights; and the explanation of how these forms have 

 been produced is not a theory only, but has been observed 

 in actual operation in the accessible portions of manv 

 glaciers. Rocks and stones are to be seen embedded in 

 the ice and actually scratching, grooving, and grinding the 

 rock beneath in their slow but irresistible onwai'd motion. 

 The rocky islets in Windermere, Ullswater, and other 

 lakes, as well as the Thousand Islands of the St. Lawrence, 

 are thus ice-ground ; and the amount of the grinding can 

 often be seen to be proportional to the pressure and motion 

 of the advancing glacier. I recently noticed in the marshy 

 alluvial plain above Derwentwater a projecting rock which 

 has been ground down to so regular a curve as to look like 

 a portion of an enormous globe buried in the earth. By 

 rough measurement and estimate this rock was about 

 250 feet across, and 20 or 30 feet high. It was formed of 

 hard slate, with numerous quartzite veins, the whole 

 ground down to a uniform spherical surface. It had 

 evidently once been an island in the lake, having a much 

 broader base now hidden by the alluvium, and may origin- 

 ally have been one of those abrupt craggy rocks a few 

 hundred feet high, which, owing to their superior hardness 

 or tenacity, resisted ordinary denudation, and which, when 

 above the old ice-level, form those numerous " pikes " which 

 add so much to the wild and picturesque scenery of the 

 district. Looking at such rocks as this, with outlines so 

 utterly unlike any that are produced in similar formations 

 by sub-aerial denudation — and they are to be seen by 

 scores in all glaciated regions — we cannot but conclude 

 VOL. I. F 



