58 Transactions of the Canadian Institute. [vol. x. 



western Ohio and Indiana. The ice stream which flowed westward and 

 southwest ward through the basins of Lakes Ontario and Erie was one 

 of the greatest currents of the ice sheet. The southwestern peninsula 

 of Ontario lay between this stream and another like it which crossed 

 Georgian Bay and moved southward through the basin of Lake Huron, 

 turning toward the southwest to join with the Ontario-Erie stream in 

 northwestern Ohio. The culminating point of the highlands forms the 

 promontory or " mountain " west of Collingwood. This highland ob- 

 structed the flow of the ice and was overflowed by it for a relatively 

 brief time, as compared with the lower lands around it. This is why 

 ice lobes projected forward from the mean line of the ice front in the lake 

 basins and re-entrants reached back from that line on the highlands. 

 The larger elements of relief gave the ice front its lobate form. 



The Manner of the Glacial Retreat. 



One of the peculiarities which characterised the retreat of the 

 Wisconsin ice sheet was the oscillation of its front or edge. During the 

 time of its general retreat its front did not retreat evenly nor at a uniform 

 rate, but by alternating and recurring steps of advance and retreat, in 

 which the backward steps were always longer than the forward steps. It 

 was as though the retreating ice front underwent continual oscillations 

 in which it took two steps backward and one forward over and over 

 again, the result being that, on the whole, the front of the ice retreated 

 in a northerly direction. These are known as the stadial oscillations of 

 the ice front. Other oscillations subordinate to the stadial oscillations 

 are known as minor oscillations. 



The amplitude of the stadial oscillations, by which is meant the 

 distance or space over which the ice front retreated and readvanced in 

 each complete oscillation, varied considerably under different conditions 

 and has been determined with only approximate accuracy in a few cases. 

 We know that in one instance at least the retreat was not less than 30 

 or 40 miles and may have been much more, and that even if the retreat 

 was only 30 miles, the readvance was not less than 20 or 25 miles. This 

 was on the "thumb" of Michigan, where the ice front had stood a little 

 farther south than the Port Huron moraine, and its next step of retreat 

 carried the front back northward from the "thumb" far enough to open 

 a relatively wide passage between Saginaw Bay and the south part of 

 Lake Huron. Just before this time the ice front rested at such an 

 altitude on- the "thumb" that the lake waters in the basin of Lake Erie 

 were held up to an altitude about 80 feet higher, for the lake level fell 

 this much in consequence of this step of retreat and the passage opened 



