THE ICE AGE AND ITS WORK. 685 



of the grinding can often be seen to be proportional to the pres- 

 sure 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 two hundred and 

 fifty feet across, and twenty or thirty 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 originally have been one of those abrupt 

 craggy rocks a few hundred feet high, which, owing to their su- 

 perior 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 subaerial 

 denudation and they are to be seen by scores in all glaciated re- 

 gions we can not but conclude that the ice tool has done more 

 than merely rub off the angles and minor prominences, and that 

 it has really ground away rocky hills to an unknown but very 

 considerable extent ; and this conclusion is, as we shall see, sup- 

 ported by a very large amount of confirmatory evidence. It may 

 be noted that ice-ground rocks usually show the direction in 

 which the ice has moved, by the side opposed to the motion being 

 more completely smoothed than the lee side, which often retains 

 some of its ruggedness, having been protected partly by the ice 

 overriding it and partly by the accumulation of its own debris. 

 Where such rocks occur in the higher parts of valleys the smooth 

 side always looks up the valley from which the glacier has de- 

 scended. In the more open parts of valleys, or in high coombs or 

 cirques, where two or more small ravines meet and where the ice 

 may have been embayed and have acquired a somewhat rotary 

 motion, the rocks are seen to be ground down on all sides into 

 smooth mammillated mounds or hummocks, showing that the ice 

 has been forced into all the irregularities of the surface. An ex- 

 ample on a small scale is to be seen in Cwm Glas, on the north 

 side of Snowdon, above the fine moraine already mentioned, and 

 in many other places around the same mountain. On the whole, 

 considering their abundance in all glaciated regions, and the 

 amount of information they give as to the direction and grinding 

 power of ice, these rounded rocks afford one of the most instruc- 

 tive indications of the former presence of glaciers ; and we must 

 also agree with the conclusin of Darwin (in a paper written after 

 studying the phenomena of ice-action in North Wales, and while 

 fresh from his observations of glaciers and icebergs in the South- 



