THE GLACIAL PERIOD. 17 7 



exhibit parallel striae, ruts, and grooves, often in great perfection. 

 It is also easy to tell from them in what direction the ice has 

 moved ; for it is the side facing that direction which shows the 

 most marked glaciation, and which, in allusion to the severe abra- 

 sion it has experienced, is called by the Swiss geologists the Stoss- 

 seite (lit. pushing- or thrusting -side). The leeside {Lee-seite) 

 of roches moutonne'es is generally less rubbed and worn, and, in 

 many cases, is even quite rugged and free from glacial markings. 

 Standing at the head of a valley in the Scottish Highlands and 

 looking down, the hill-sides on either hand present a somewhat 

 smooth and undulating surface, an appearance which vanishes 

 when we walk down the valley and then turn to look back. The 

 rocks which seemed rounded off when we viewed them from the 

 upper reaches of the valley, now assume a much more broken 

 and rugged aspect, a phenomenon which must be attributed to 

 precisely the same cause as that which produced the planed 

 and striated surfaces and the roches moutonne'es. The observer in 

 such a mountain -region soon becomes aware also of another 

 appearance which is sufficiently remarkable. In some of our 

 higher mountain -valleys he sees the striated rocks and the 

 smoothed hill-slopes extending for a considerable distance up- 

 wards until they reach a certain elevation, above which the 

 mountains show no traces of abrasion, but all is harsh and 

 severe. Keflecting upon these facts, he concludes that the val- 

 leys have been at some period filled to a less or greater depth 

 with ice, which flowed down towards the low grounds, smoothing 

 and striating the rocks, removing asperities, and producing 

 roches moutonne'es after the very same manner as the glaciers of 

 Switzerland and Norway. And by measuring the height to 

 which the glaciated rocks extend, he is enabled to form an 

 estimate of the thickness attained by the ice. By following out 

 similar observations, we have now not only ascertained the 

 thickness of the ice and the direction in which it flowed, but 

 we have also acquired some definite notion of the degree at 

 which its upper surface sloped away to the horizon. 



All the valleys of the Highlands and Southern Uplands 



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