CHAP. IX ANCIENT GLACIAL EPOCHS 173 



years to remove one foot ; while the Po is the most rapid, 

 taking only 729 years to do the same work in its valley. 

 The cause of this difference is very easy to understand. 

 A large part of the area of the Mississippi 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 rain- 

 fall, 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 coin- 

 cide 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 and 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 downward 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 comparatively 

 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. 



