174 A. p. COLEMAN LAKE IROQUOIS AT TORONTO 



It may be supposed that the interglacial relief from pressure allowed the 

 northeast part of the continent to rise, thus damming back the waters of 

 the Scarboro lake. Ultimately, however, the rise may have been suffi- 

 cient to cause the formation of fresh snowfields and an ice-sheet, loading 

 down Labrador and Quebec once more till the region sank under its 

 burden and permitted the Scarboro basin to be drained again. If this 

 theory is correct, the ice must have advanced southwest very slowly, for 

 after the lowering of the northeast end of the lake sufficient time elapsed 

 to allow the interglacial valleys to be cut down to a base even lower than 

 the present one. It should be added, however, that the thickness of drift 

 materials cut through was not much over half the average thickness of 

 the beds in which the present Don river is carving its valley. 



Watkr-levels during last Advancf: of the Ice 



The cutting of the interglacial valleys no doubt required a long time, 

 perhaps thousands of years, but at length the ice-front advanced far 

 enough to fill the lower end of the Ontario valley, ponding back the water 

 to a far higher level than we have any record of before, since stratified 

 sands and clays are found at Scarboro and north of Toronto at least 320 

 feet above lake Ontario. 



At the former point there are four well defined beds of till with strati- 

 fied materials between, the whole series of deposits reaching a thickness 

 of 200 feet. It is very probable that when these beds were formed the 

 ice-front crossed the basin of lake Ontario diagonally just northeast of 

 Toronto, sometimes advancing and at others retreating, the water rising 

 high along its face. It would be of great interest to determine whether 

 any equivalent series of high leve^, stratified and unstratified, glacial beds 

 exists in the state of New York to the east or southeast of Scarboro. 



It is probable that during this last invasion of the ice the region stood 

 much lower than now, so that the stage of apparent high water was not 

 actual but only relative to the present level. The land surface may even 

 have been 570 feet lower than now, so that but for the intervening mass 

 of ice it might have been flooded by the sea. No fossils have 3'et been 

 found in these beds at Scarboro to determine the character of the water, 

 but in an interglacial bed of stratified sand three and a half miles north 

 of Toronto ba}^ overlying the till sheet which covers the Toronto forma- 

 tion, several species of fresh water shells occur 220 feet above lake Ontario. 

 These sands rise 27 feet higher and have a thin bed of till interstratified 

 with them near the top. It is probable that this deposit is equivalent in 

 age to part of the interstratified sands between the u})per layers of till at 

 Scarboro, which Avould indicate fresh water in the lake of the time. 



