20 TRACES OF THE GREAT ICE AGE IN DERBYSHIRE. 



rhinoceros, roamed over the now fertile ground. Professor 

 James Geikie recognizes no fewer than four of the interglacial 

 periods, each one characterized by a retreat of the glaciers, and 

 a return to temperate conditions. '■' These alternations were 

 accompanied by oscillations of the sea level. 



But — and especially since this Society is Archaeological rather 

 than Geological— it will not be out of place if we digress slightly 

 in order to describe very briefly the phenomena accompanying 

 the passage of a glacier over its rocky bed. The problems we 

 are discussing can only be attacked in the light of our knowledge 

 of the phenomena of glacial action taking place at the present day. 



We may first of all place on one side the idea that glaciers 

 have scooped out the valleys down which they flow. That 

 glaciers do possess considerable power of eroding even hard 

 rock there can be no doubt, but that they have produced the 

 valleys down which they flow, or have flowed, there is no evidence 

 to prove ; those were almost invariably produced in the flrst 

 place by the action of running water, but may have been 

 deepened by glacial action. Considerable light is thrown on 

 the glacial phenomena of our own country by a study of 

 what is taking place at the present time in a glacier district 

 such as that of the Alps. In many cases the Swiss glaciers 

 (e.g., the Rhone) have receded greatly in recent years, and one 

 can readily observe the action of the ice on the rocky bed over 

 which it has flowed. In the first place it rounds and polishes 

 the rock, and also produces a number of grooves and striae, which 

 are more or less parallel to the direction of motion of the 

 ice. Where large bosses of rock lie in the path of the glacier, 

 the side upon which the glacier impinges is worn down until it 

 presents an inclined plane, the opp )site side being as a rule more 

 craggy — a phenomenon known as "crag and tail." It is not 

 difficult to see how all this comes about. The detrital material 

 perpetually falling from the cliffs, and rolling down the slopes 

 (and which, in a river valley would fall into and be carried away 



* " Fragments of ?',arth Lore," p. 321. 



