/ 
water. In deformation-mountains of recent age, we naturally 
expect to meet with cascades and,waterfalls, for the streams 
and rivers of such a region are relatively young. They have 
only, as it were, commenced the work of erosion. But plains 
and plateaus of erosion which have existed for ages as dry 
land, and in which a complete hydrographic system has long 
been established, should show no great waterfalls. Yet we 
find cascades and waterfalls more or less abundantly developed 
in all the plains and plateaus of Northern Europe, and the 
corresponding latitudes of North America; and most of these 
lands are of very great antiquity, their main lines of drainage 
having been established for a long time. Obviously, the 
hydrographic systems have been disturbed, and the disturbing 
element has been glacial action. During the Ice Age the 
long-established pre-glacial contours were greatly modifed. 
Frequently, indeed, the minor valleys in plateaus and plains 
were obliterated, while even the main valleys were often 
choked with debris. When glacial conditions passed away, 
and streams and rivers again flowed over the land, they could 
not always follow the old lines of drainage continuously, but 
were again and again compelled to leave those, and to cut out 
new courses in whole or in part. Hence the frequent occur- 
rence of cascades and waterfalls in formerly glaciated lands. 
Another cause for the existence of waterfalls in long- 
established drainage-systems must be sought for in crustal 
disturbances. In general, deformations of the crust would 
seem to have been very gradually brought about, so gradu- 
ally, indeed, that they have often had little or no influence 
upon the courses of great rivers. Anticlines slowly develop- 
ing across a river-valley have been sawn through by the river 
as fast as they arose. Dislocations, in like manner, would 
i seem to have been very slowly developed. Frequently these 
} have traversed a river-valley without in any way disturbing 
the drainage, the rate of erosion having been equal to that 
of displacement. On the other hand, we know that faulting 
or dislocation may sometimes be effected suddenly. Werea 
| fault to be developed across a river-valley either suddenly or 
at a greater rate than the rate of erosion, and were its down- 
throw to be in the direction to which the river flowed, a 
waterfall would certainly be the result. . 
418 STRUCTURAL AND FIELD GEOLOGY 

