CHAP. IX.] 



ANCIENT GLACIAL EPOCHS. 



1G7 



basin consists of the almost rainless prairie and desert regions of 

 the west, while its sources are in comparatively arid mountains 

 with scanty snow-fields, or in a low forest-clad plateau. The 

 Po, on the other hand, is wholly in a district of abundant rainfall, 

 while its sources are spread over a great amphitheatre of snowy 

 Alps nearly 400 miles in extent, where the denuding forces 

 are at a maximum. As Scotland is a mountain region of rather 

 abundant rainfall, the denuding power of its rains and rivers is 

 probably rather above than under the average, but to avoid any 

 possible exaggeration we will take it at a foot in 4,000 years. 



Now if the end of the glacial epoch be taken to coincide with 

 the termination of the last period of high excentricity, which 

 occurred about 80,000 years ago (and no geologist will consider 

 this too long for the changes which have since taken place), it 

 follows that the entire surface of Scotland must have been since 

 lowered an average amount of twenty feet. But over large areas 

 of alluvial plains, and wherever the rivers have spread during 

 floods, the ground will have been raised instead of lowered ; and 

 on all nearly level ground a^id gentle slopes there will have 

 been comparatively little denudation; so that proportionally 

 much more must have been taken away from mountain sides 

 and from the bottoms of valleys having a considerable down- 

 ward slope. One of the very highest authorities on the subject 

 of denudation, Mr. Archibald Geikie, estimates the area of these 

 more rapidly denuded portions as only one-tenth of the com- 

 paratively level grounds, and he further estimates that the 

 former will be denuded about ten times as fast as the latter. It 

 follows that the valleys will be deepened and widened on the 

 average about five feet in the 4,000 years instead of one foot ; 

 and thus many valleys must have been deepened and widened 

 100 feet, and some even more, since the glacial epoch, while 

 the more level portions of the country will have been lowered 

 on the average only about two feet. 



Now Dr. Croll gives us the following account of the present 

 aspect of the surface of a large part of the country : — 



" Go where one will in the lowlands of Scotland and he shall 

 hardly find a single acre whose upper surface bears the marks 

 of being formed by the denuding agents now in operation. He 



