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 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 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 
