402 GEOLOGY. 



With further stages of ice retreat, the outlet was let down to the Chain- 

 plain arm of the sea presently to be noted. By this time Lake Algon- 

 quin had given place to the great Nipissing Lakes (Fig. 522), which 

 had their outlet via Lake Nipissing to the Ottawa, and thence to the 

 Champlain arm of the sea. Subsequently the outlet was shifted to 

 its present position, probably by gentle warpings of the surface. 1 



Without doubt similar complicated lake histories attended the 

 retreat of the ice in the Mackenzie and Hudson Bay basins, but little 

 is yet known regarding them. 



A very important lake was also formed in the Red River valley 

 of the north (Lake Agassiz), discharging in its earlier history, into the 

 Minnesota river at Lake Traverse. As Lake Agassiz was not con- 

 nected with the complex system of basins of the St. Lawrence valley, 

 it had a comparatively simple history. It grew to the northward with 

 the retreat of the ice which held it in at that end, and continued to dis- 

 charge into the Minnesota river at Lake Traverse, cutting down its 

 outlet and forming a series of beaches about its borders, until the retreat 

 of the ice enabled it to find a northerly outlet in some position yet un- 

 known. While discharging by this northerly outlet, it made another 

 set of beaches. On the further withdrawal of the ice, its waters were 

 discharged, and the lake became extinct. Lakes Winnipeg and Winni- 

 pegosis may be regarded as its diminutive successors in a sense, but 

 they are rock-bound or earth-bound lakes, while Lake Agassiz was 

 ice bound on its northerly border. 2 Multitudes of smaller lakes came 

 into existence in the regions of strong relief as the ice withdrew. Their 

 histories are for the most part less complicated. Few of them have 

 been studied in detail. 



It is probable that there were corresponding lacustrine sub- stages 

 at the close of each of the several glacial epochs, but their history has 

 not been worked out, and because of the overriding of later ice, will 

 probably never be deciphered in detail. 



The evidence which demonstrates the existence of these expanded 

 lakes is found chiefly in the deposits which they made, and in the 

 topographic features which they developed about their shores. Many 

 of the former shore-lines have been traced in detail, and most of them 



1 An account of the history of the Great Lakes, by- F. B. Taylor, is found in 

 Studies in Indiana Geography. 



2 The glacial Lake Agassiz, Upham, Mono. XXV, U. S. Geol. Survey, 1895. 



