200 SURFACE GEOLOGY. 
which they mark. That they are nothing else than ancient 
lake beaches we shall hope to prove farther on. 
In the southern half of the Mississippi valley the evidences 
of glacial aetion are entirely wanting, and there is nothing 
corresponding to the wide-spread Drift deposits of the north. 
We there find, however, proofs of erosion on a stupendous 
scale, such as the valley of East Tennessee, which has been 
formed by the washing out of all the broken strata between 
the ridges of the Alleglianies and the massive tables of the 
Cumberland Mountains, — the caüons of the Tennessee, one 
thousand six hundred feet deep, etc. Here also, as in the 
lake. basin, the channels of excavation pass far below the 
. deep and quiet waters of the lower rivers; proving by their 
depth that they must have been cut when the fall of these 
‘rivers was much greater than now. 
The history which I derive from the facts cited above is 
briefly this : 
lsr.— That in a period probably synchronous with the 
glacial epoch of Europe, —at least corresponding to it in the 
sequence of events,—the northern half of the continent of 
North America had a climate comparable with that of Green- 
land; so cold, that wherever there was a copious precipita- 
tion of moisture from oceanic evaporation, that moisture was 
congealed and formed glaciers which flowed by various routes 
towards the sea. 
2ND.—-That the courses of these ancient glaciers corres- 
ponded in a general way with the present channels of drain- 
age. The direction of the glacial furrows proves that one 
of these ice rivers flowed from Lake Huron, along a channel 
now filled with drift, and known to be at least one hundred 
and fifty feet deep, into Lake Erie, which was then not a 
lake, but an excavated valley into which the streams of 
Northern Ohio flowed, one hundred feet or more below the 
present lake level. Following the line of the major axis of 
e Erie to near its eastern extremity, here turning north- 
east, this glacier passed through some channel on the Cana- 
