SURFACE GEOLOGY. 5 



local causes, and do not hold a definite place in the sequence of Drift 

 phenomena, but much interest attaches to them, and they will be de- 

 scribed somewhat in detail in another part of this chapter. 



9th. Above all the drift deposits of the lake basin, and more recent 

 than any of them, are the " Lake Ridges ;" embankments of sand, gravel, 

 and clay, which run imperfectly parallel with the present margin of Lake 

 Erie. Of these the lowest is about 100 feet, the highest some 250 feet, 

 above the present level of the lake. In New York, Canada, Indiana, 

 and Michigan a similar series of ridges has been discovered on the slopes 

 of the basin of the great lakes ; and they have every where been accepted 

 as evidence that the water of the lakes once reached the level of the 

 highest ridge, and that the lower ones mark successive periods of rest in 

 its descent. 



In the southern half of the Mississippi valley the evidences of glacial 

 action are entirely wanting, and there is nothing among the superficial 

 deposits corresponding to the wide-spread Drift 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 Alleghanies and the massive 

 tables of the Cumberland Mountains, the canons of the Tennessee, 1600 

 feet deep, etc. Here, also, as in the lake-basin, the channels of excava- 

 tion pass 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 deduce from the facts cited above is briefly this : 



1st. At a period probably synchronous with the glacial epoch of 

 Europe — at least corresponding to it in sequence of events — the northern 

 half of the continent of North America had an arctic climate ; so cold, 

 indeed, that wherever there was a copious precipitation of moisture from 

 oceanic evaporation, that moisture fell as snow ; and this, when consoli- 

 dated, formed glaciers which flowed by various routes toward the sea. 

 These glaciers, in the approach and retirement of the period of greatest 

 cold, were local. During the prevalence of the extremest arctic condi- 

 tions they were general, so far as this, that a great ice-sheet moving 

 from the north north-west covered all New England, and other great 

 glaciers occupied the region east of the Mississippi and north of the Ohio. 



2d. The courses of these ancient local glaciers correspond in a general 

 way with the present channels of drainage, and we owe to them some of the 

 most striking features of the topography of the Western States, where the 

 geological structure is simple, and the topography was once exceedingly 

 monotonous. By local glaciers, flowing down from the Canadian high- 



