298 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



in a stadium and a stadial moraine marks the forward climax of one 

 of these periods. The readvance to each stadial moraine covered a 

 very considerable distance, certauily in some cases 25 to 50 miles and 

 perhaps twice as much. The long mtervals of time betv/een these 

 moraines account perhaps to some extent for the commonly observed 

 discordant relation between them. Each stadium m turn is marked 

 at periodic intervals by weaker moraines which are superposed upon 

 the stadia. These mark subsidiary halts or perhaps slight read- 

 vances, and there is a numerous series of them in each stadium. 

 There was, however, only a slight movement of readvance associated 

 with these minor stadial moraines, and they record less strongly 

 marked and shorter periodic climatic variations. 



The most important oscillatory variation in the retreat of the ice, 

 so far as its effect on the lake history is concerned, is the stadial 

 oscillation. Its relatively long period and the wide space covered each 

 time by the movement of readvance make it the chief cause of com- 

 plexity, for after lakes were lowered by a retreat of the ice the stadia! 

 readvance was likely to close the outlet last opened and reelevate tho 

 lake to a higher level. Three times, certainly, and perhaps four or 

 live times, this kmd of change affected the waters of the Huron-Erie- 

 Ontario basm. During several oscillations the ice front stood in 

 critical relations to the land barriers that held up the lakes and changes 

 of relatively slight amount in its position opened or closed outlets 

 and changed the level of the lake waters. 



Considering the apparent character of the glacial oscillations, it 

 seems necessary to take account also of the halts of the ice front at 

 the backsteps or climaxes of retreat. These probably affected the 

 lakes and lasted as long as did the halts on the stadial moraines 

 which mark the climaxes of advance. In one case at least there is 

 remarkably clear and complete proof of the long duration of a par- 

 ticular lake stage which existed during the pause at a backstep or 

 climax of retreat. 



BRIEF OUTLINE OF THE GREAT LAKES HLSTORY DURING AND AFTER 

 THE RECESSION OF THE LAST ICE SHEET. 



THE FIRST SMALL LAKES. 



As soon as the ice front Avithdrew to the north side of the southern 

 watershed of the Great Lakes, small ice-dammed lakes began to be 

 formed. In Ohio a number of such lakes appeared along the north 

 side of the divide south of Lake Erie. It seems a necessary inference 

 that in consequence of the stadial oscillations of the ice many of these 

 earliest small lakes were first formed at an extreme position of retreat 

 and then overridden and obliterated by the next readvance. Indeed, 

 in some cases this may have occurred two or three tunes before the 



