1913] The Moraine Systems of Southwestern Ontario. 59 



around the north side of the "thumb" was wide enough to allow heavy 

 storm waves to make strong beach ridges. Then when the 

 ice readvauced it closed this passage and pushed up on to the "thumb" 

 far enough to raise the lake waters about 45 feet. At the climax of this 

 readvance the ice front rested for a relatively long time on nearly the 

 same line and it was during this time that the Port Huron moraine was 

 built. Not that the ice itself was stationary, for it was not. The ice 

 was always moving slowly forward, but it was also melting. The melting 

 of the ice always tended to drive the front back and it was only when the 

 rate of melting exactly balanced the rate at which the ice advanced that 

 the front became, as we say, stationary. At these times the ice front 

 paused or halted, though the ice itself kept moving, and it was only 

 during these times that terminal or marginal moraines were built. 

 Whether the ice front at any given time or place retreated or advanced 

 or stood in a stationary state depended upon the ratio between melting 

 and the forward movement of the ice. If melting did not take place 

 there was no loss of ice and the front advanced; if melting took place 

 faster than the ice advanced, then more ice was lost in a given time than 

 came forward and the front retreated. 



Whenever the ice front halted a marginal deposit of some kind was 

 made, for the ice nearly always carried in its lower layers more 

 or less dirt or detritus gathered from the surface of the ground or of the 

 rock over which it moved. This detritus comprised all grades of coarse 

 and fine rocky materials, and they were mixed promiscuously together. 

 Since a moraine of some kind was always made when the ice front halted 

 for any length of time, we are compelled to believe that moraines were 

 built at halts following movements of retreat as well as at those following 

 movements of advance. But the moraines formed at climaxes of retreat 

 were always overridden and obliterated at the next advance. From 

 this fact it follows that the moraines which we see and study were made 

 at successive climaxes of readvance during a general movement of retreat. 

 This record of the glacial retreat furnishes by far the greater part of the 

 material available for the study of the manner of glacial movements 

 and for the study of the origin of the drift forms which make up the greater 

 part of the surface. Southwestern Ontario is covered with a series of 

 these terminal moraines, all made at climaxes of readvance during the 

 general recession. That some movements of retreat and advance covered 

 more distance than others and were more important in their significance 

 is not to be doubted, but only one or two distinctions of this kind have 

 been made at the present time. 



The outline of the ice front changed greatly with the progress of 

 retreat. At first the whole region was covered and there were no lakes in 



