112 N. H. AVINCHELL — GLACIAL LAKES OF MINNESOTA 



in determining the present general topography, in some such manner as 

 the rock}^ substructure imprints itself on the present topography, but 

 the legible record with which we have to do in studying the glacial lakes 

 is emi)hatically that of the Wisconsin epoch, the last which visited 

 Minnesota. 



Of these two ice-lobes, that which departed latest from the northern 

 part of the state was the northeastern. As the examination of the state 

 has progressed the recognition of the lake Superior ice-lobe has become 

 more and more pronounced and important. The extent of the westward 

 movement of drift material b}'' this ice-lobe has been traced farther and 

 farther to the west. At the same time the earlier disappearance of the 

 northwestern ice-lobe and its line of retreat rather toward the east-north- 

 east — at least from the extreme northern part of the state — have been 

 prett}' well established. It appears that preci})itation sufficient for the 

 support of these ice-masses was maintained longer to the north and east 

 of lake Superior than in the plains of the Saskatchewan, and it becomes 

 probable that both ice-lobes sprang from the Laurentide birthplace and 

 were differentiated in direction of flow and in other features after they 

 were gathered into the great valleys where their separate features have 

 been separately studied. 



Two conditions have contributed to the formation of glacial lakes in 

 the state and to the ease with which they can be identified and referred 

 to their causes: (a) The northward slope of the natural surface in that 

 portion lying north of the continental watershed. (6) The duplex char- 

 acter of the drift, namely, the gray till, from the northwest, with its re- 

 sultant gray modified drift, and the red till, coming from the northeasts 

 with its characteristic red gravels and lacustrine clays. 



It was in studying the drift of northwestern Ohio, in 1870 and 1871, 

 that the writer was convinced that the ice was a barrier which i)onded back 

 the drainage into the Maumee valley, and he impressed that view on Dr 

 J. S. Newberry, the director of the Ohio survey. The lake thus formed had 

 its outlet into the Wabash valley, and has since been named lake Maumee. 

 On undertaking the Minnesota survey this theory was at once a})plied 

 to the Red River valley. It has since been widely applied in the northern 

 states, where glacial lakes have been recognized in great numbers. It is 

 probable that many more will be described. It seems almost an invaria- 

 ble law that in valle3'S sloping toward the north, and hence in general 

 toward the retreating ice-margin, glacial lakes should be formed. By 

 far the larger number of glacial lakes are found to have been due to such 

 conditions. . A small number have been due to the oblique transgression 

 of the ice-lobe across a southward sloping broad valle}', forcing the river 

 that drained such a vallev to set back in the form of a lake until its 



