204 SURFACE GEOLOGY. 
In regard to the cause of the gradual amelioration of the 
climate of the glacial epoch, by which the great glaciers of 
the lake basin were driven northward and finally altogether 
dissolved, we are not left entirely to conjecture. 
osmical causes possibly and probably had the chief agency 
in producing this result, but we have unmistakable evidence 
of at least the coóperation of another and perhaps no less 
potent cause, namely, continental depression. 
` If a cosmical cause had simply increased the annual tem- 
perature till the glaciers were all melted, without the action 
of any other agent, we should never have had the accumula- 
tion of drift deposits which now occupy all the glacial area ; 
but the drainage streams, changed in all their courses from 
ice to water, would have flowed freely and rapidly away 
through their deeply cut channels to deposit their abundant 
sediments only where their transporting power was arrested, 
in the depths of the ocean. 
Instead of this, we everywhere find evidence that this flow 
was checked, and a basin of quiet water formed by an ad- 
vance of the ocean consequent upon a subsidence of the land. 
On the Atlantie and Gulf coasts this depression progressed 
until the sea level was more than five hundred feet higher 
than now. The effect of this depression was to deeply sub- 
merge the eastern margin of the continent, and cover it with 
the “Champlain” clays. 
It is evident that at this period the drainage from the great 
water-shed of the continent must have been met by the quiet 
waters of the ocean almost at the sources of the present 
draining streams, and as the “dead water” gradually crept 
up the valleys, arresting the transporting power of their cur- 
rents, their old channels would be silted up and obliterated, 
and their valleys partially filled with materials for their sub- 
sequent terraces. In the advance and subsequent recession 
of the line of “dead water” we have ample cause for all our 
terrace phenomena. 
This continental depression accounts satisfactorily for the 
