A GEOLOGIC SPRING TIME. 279 



ued in solid rock, and is now proceeding at the rate of three 

 feet a year. 



The question has been much discussed whether the basins / 

 of the great lakes were excavated by the action of the conti- ' 

 nental glacier. By Ramsay, lake basins have been generally 

 attributed to such action. By others, the doctrine is held in 

 light esteem, since we have evidence, in some cases, that 

 glaciers have moved over sheets of clay without plowing them 

 up. I incline to agree in part with both. The positions of 

 the terminal and lateral moraines show that glaciers moved 

 along the beds of Lakes Erie, Huron, and Michigan, and Sag- 

 inaw and Green Bays. What directed ice-streams to these 

 positions? A pre-existing valley. What caused the valley? 

 The erosion of the great river which had been flowing out of ) 

 Lake Superior during Mesozoic and Caenozoic ages. The val- / 

 ley may have been a mile, or five miles, wide, and bounded 

 by precipitous rocky walls. When the glacier commenced its 

 movement along such a valley, it exerted powerful erosion 

 along the steep bounding walls, and wore them down to the 

 gentle slopes which now form part of the bed of the lake. 

 The basins of the lakes are demonstrably works of erosion. 

 Why the great glaciers worked there more than elsewhere, 

 was because the great river had inaugurated the work and in- 

 vited the glacier. A glacier also moved out of the western 

 end of Lake Superior. A valley already existed indeed a 

 lake basin existed, shaped by the ancient upheaval of rocks 

 along the northern and southern shores. 



In regions where returning spring-time found the general 

 surface nearly level, and locally indented with basin-like 

 depressions, the Champlain floods formed large numbers of 

 lakes and lakelets. Such depressions might arise from the 

 rocky configuration of the country especially the larger de- 

 pressions. More generally they were mere intervals inclosed 

 by hills and ridges of Drift. Thus arose the numerous lakes 

 of Maine, Michigan, and Minnesota. Whenever a lakelet 

 found an outlet, the process of erosion began ; the lakelet was 

 continually lowered, and in many cases, it has been completely 



