558 



CENOZOIC TIME. 



of the land over the higher latitudes, — that is, of nearly the same area 

 that was depressed in the Champlain period. As the Champlain de- 

 pression was greatest to the north, so it was with the elevation follow- 

 ing it ; for the height at which the Champlain deposits now stand 

 over the continent, from the southern Drift limit to the Arctic, is a 

 consequence of this elevation. Terraces exist along all the rivers and 

 about all the lakes of the North American continent, excepting its 

 more southern portions ; and these were a necessary consequence of 

 the changes of level, and are testimony as to the amount of this change, 

 and the way in which it went forward in different regions. 



1. Terraces. — The connection of the existence of terraces with an 

 elevation of the land is illustrated in the following figures. Fig. 943 

 represents a section of a river valley filled up with the stratified Drift 

 of the Champlain period, and having its narrow river-channel R, and 



Fig. 943. 



Section of a valley in the Champlain epoch, with dotted lines showing the terraces formed in con- 

 sequence of an elevation of the land. 



its broad river-flat f f, either side of the channel. Rivers in an 

 open country have always both these two elements, a channel and a 

 river-jiat or flood-plain. The stream occupies the former during 

 ordinary low-water, but spreads over the latter during freshets. The 

 sweeping violence of the flood determines the limits, other things 

 being equal, of the flood-plain or river-flat. 



If now the interior of a continent be raised a hundred feet higher 

 than along the sea-coast, the river will have an increased angle of 

 slope, a quicker flow, and greater power of erosion ; and it will grad- 

 ually wear down its channel, if there are no rocks to prevent, until 

 the old slope is again attained. The flood-plain will also sink at the 

 same rate, although with more or less changed limits, according to 

 many causes of variation ; among which causes a diminution in the 

 amount of water, from any cause, would make itself apparent along 

 the whole course of the stream. After such an elevation, the level d d' 

 might be the flood-plain or river-flat. After another similar eleva- 

 tion, o V might be the flood-plain and channel. 



Similar effects would ultimately be produced from an equal eleva- 

 tion of the whole region, from the coast to the head of the stream, 

 provided the slope of the surface below the coast-line were decidedly 

 more rapid than the average pitch of the river channel above it. 



