REVIEW OP GEOLOGICAL STRUCTURE. 45 



on the old Cuyahoga delta at Cleveland — where no moraine could be pro- 

 duced by an ice sheet without, more or less, disturbing the beds below. 



Fifth. Precisely similar ridges are now being formed in many places 

 along the shores of the present lakes : wherever, indeed, the shore is 

 sloping and composed of loose material. Excellent examples of these 

 surround the head of Lake Michigan, where the higher and older beaches 

 may be directly compared with the lower one, and shown to have the 

 same character and history. 



ORIGIN OF THE BASINS OP THE LAKES. 



The subject of the origin of the basin of Lake Erie, and, as illustra- 

 tive of that, the origin of all our great lakes, was discussed at some 

 length in the chapter on Surface Geology of Volume I. After a review 

 of the facts presented by the present condition of the lake-basins, it was 

 shown that the history of the formation of the lakes was briefly as 

 follows : 



First. That all the lake region has been above the sea level since the 

 close of the Carboniferous age ; no great disturbance has affected it, but 

 its topography has been greatly modified by surface erosion, and hence 

 its system of drainage has suffered important changes, such as the trans- 

 fer of the drainage of some portions of the lake region — first, from the 

 Mississippi to the Hudson, and then from the Hudson to the St. 

 Lawrence. 



Second. That, previous to the ice period, the eastern half of the con- 

 tinent was higher above the ocean than now, and the rivers of this 

 region had so long flowed in their channels as to cut them much below 

 the beds of the present streams ; that at this time the area occupied by 

 Lake Erie was traversed by a river which had cut its bed more than two 

 hundred feet below the present surface of the lake ; and that Grand 

 River, the Cuyahoga, the Maumee, and other streams were tributaries of 

 this river, flowing through gorges of nearly equal depth with its own. 



Third. That the excavation of all the lake-basins was chiefly accom- 

 complished by local glaciers descending from the Canadian highlands, 

 either in the advance or the retreat of the great glacier, or both — following 

 and greatly broadening and deepening the old drainage channels, or both. 



Fourth. That the local glaciers which formed the lake basins moved 

 along the lines of their major axes ; those which formed Lake Ontario 

 and Lake Erie moving from the north-east, the Lake Huron glacier from 

 the north, the glaciers of Lake Michigan and Lake Superior moving 

 towards the south, south-east, and south-west. 



The arguments advanced to support these views were — 



First. That the lake basins are boat-shaped cavities, deeply exca- 

 vated, in nearly horizontal strata, by some eroding agent. 



