igoo-i.] Physical Geology of Central Ontario. 167 



plication, not even at the crossing of the old depressions, and the till 

 sheet descends at the opposite side of the section. Some twelve miles 

 further west it again ascends over plicated beds. In some sections east 

 of Scarboro' the till sheet is seen in clear cut cross section ascending 

 across the beds with almost no plication of the underlying deposits 

 (Plate II). 



Whether the three periods of glacial transgression and retreat, 

 marked by the three till sheets and the intermediate deposits in Central 

 Ontario, are to be correlated with similar periods as determined to the 

 south of the lake, or represent local variations in the later positions of 

 one ice sheet, it is at present impossible to say. The correlation of the 

 deposits in the two localities, for lack of sufficient knowledge of inter- 

 vening areas, is not yet definitely determined. Professor Chamberlin 

 has provisionally classified the fossiliferous beds beneath the middle till 

 sheet as contemporaneous with the interval preceding the Wisconsin 

 formation, regarding the middle till sheet of the Toronto sections as 

 equivalent to the Wisconsin till ('95b, 273). He suggests that the 

 Toronto beds might lie in a position at least one hundred miles back 

 from the front of the ice sheet whose till deposits overlie them ( '95a, 

 768 ; see also, Coleman, 1900). 



A feature of particular interest is the fact that here are two sheets of 

 glacial till, overlying still soft sands and gravels, over which the ice that 

 deposited the till sheets must have transgressed. In its transgression 

 the ice sheet has passed over large areas without leaving any mark of 

 disturbance in the underlying beds. In some cases, not in all, where it 

 ascended, the beds on the side from which the ice came are very much 

 disturbed, but the disturbance is confined to the place of ascent. Many 

 instances of modern glaciers over-riding soft deposits have been cited as 

 evidence of the inability of the ice to do significant erosive work. To 

 this the principal objection has been that this inability is shown only at 

 the edge of the sheet. In Central Ontario, whatever the distance 

 between these beds and the edge of the ice sheet which overlay them 

 may have been, it is extremely improbable that at its maximum exten- 

 sion they were just at the margin. There were two periods when the 

 ice overran obviously very incoherent deposits, and there is no known 

 evidence of great erosion by these ice sheets alone, over a distance of 

 more than one hundred miles in length, and of a width undetermined, 

 but more than six miles for the middle till sheet, and over an area very 

 much larger for the upper, and perhaps for both. Whatever may have 

 been the amount of material eroded by the ice during these two 

 advances, there is an enormous amount still in place, lying between the 



