6 BULLETIN 141, U. S. DEPARTMENT OF AGRICULTURE. 



to obliterate the previous relief of the country and through the depo- 

 sition to unequal thickness of the different classes of glacial materials. 



The recession of the glacial ice during this latest state of glacia- 

 tion undoubtedly occupied a long period of time as measured in 

 years. It was accompanied by the formation of large volumes of 

 water derived both from the melting of the ice and from the 

 normal annual precipitation over the region. This water was forced 

 to seek outlets to the sea through new channels cut across the 

 moraines and till plains of the north-central States and through 

 channels formed along the border between the ice and a higher lying 

 upland which had been freed from its ice covering in the north- 

 eastern States. During the progress of the establishment of these 

 new drainage ways there was an extensive ponding of the glacial 

 waters within the lower lying areas included between the ice front 

 and the elevated land areas of different characteristics which lay to 

 the southward. This stage of deglaciation resulted in the forma- 

 tion of several large bodies of glacial lake waters along the southern 

 limits of the present Great Lake system and of innumerable small 

 glacial lakes held between the more elevated moraines and in depres- 

 sions in the till planes of the glaciated upland. 



The larger glacial lakes occupied successively lower levels along 

 the ice front as the ice retreated, since successive channels opening 

 to the sea were uncovered during the long period of glacial recession. 

 Owing to this frequent change in level the students of glacial geology 

 have been able to identify a large number of different glacial lakes 

 which have been given different names. In some instances as man^y 

 as 10 or 12 different stages of glacial lake occupation, possessing dif- 

 ferent outlets and giving rise to characteristic glacial lake sediments 

 at many levels, have been identified. 



For the purposes of the discussion of the glacial lake deposits which 

 ultimately gave rise to the soils of the Clyde series it will hardly be 

 necessary to outline the different areas occupied by even the larger 

 successive glacial lakes. It will be sufficient instead to outline in a 

 general way the largest areas within which these deposits were 

 formed, to indicate the sources of the material which gave rise to 

 this particular group of soils as distinct from other soils of some- 

 what similar mode of formation, and to show the processes through 

 which the different soils of the Clyde series have been created. 



It is probable that the first glacial lakes formed along the border 

 of the receding ice occupied small areas around the southern ex- 

 tremity of Lake Michigan, where glacial Lake Chicago was formed, 

 and a small area extending from the vicinity of Fort Wayne, Ind., 

 eastward across the State line into Defiance, Paulding, and Van 

 Wert Counties, Ohio, where the first stage of Lake Maumee existed. 

 With a slow recession of the ice sheet the area of glacial Lake Chicago 



