igoo-i.] Physical Geology of Central Ontario. 183 



While the ice sheet was retreating across Ontario, a series of lakes 

 were formed between its front and the highlands to the south and west. 

 In the latter stages of the ice retreat, portions of the present land area 

 of Central Ontario were beneath the waters of these lakes. The land 

 was being gradually elevated at the northeastern end, so that at present 

 the old shores are not parallel with the surfaces of the existing lakes. 

 The deposits of the different periods of ice transgression and retreat 

 have been so little studied, and so little differentiation seems to have 

 been made between the deposits of sands and gravels of these periods, 

 and those formed by their re-arrangement during the periods of the 

 great Pleistocene lakes, that at present there is much confusion with 

 regard to the history of the area during the Great Lakes epoch. (Plates 

 VI and VII.) 



Recent History — A Smmnary. — Since the withdrawal of the 

 Pleistocene lakes the amount of erosion has been small. The courses 

 of the present streams are in part determined by the valleys of the 

 preglacial rivers, in part by the position assumed by the drift deposits 

 with respect to the retreating ice sheets, and in part to the controls 

 exercised by the Pleistocene-lake beach-deposits. There is at least one 

 lake (Scugog) whose drainage seems to have been affected by the 

 differential uplift indicated by the present attitude of the old lake 

 beaches.* 



Some of the streams have cut through the glacial deposits into the 

 bed rock. Streams entering Lake Ontario west of Toronto, or flowing 

 into the Georgian Bay, have cut deep steep-sided ravines and valleys 

 through drift and shale. Some few, in the vicinity of Oakville, have 

 cut deep straight-sided, flat-bottomed valleys through about forty feet 

 of drift and eighty-five feet of shale.f The present streams meander in 

 courses largely independent of their valley sides, here truncating an old 

 spur, there widening the former meander belt. Sometimes there are 

 two or three back meanders between adjacent spurs of the old valley. 

 In the upper courses, where the stream is still working upon glacial 

 debris, these misfit meanders are especially common. In the great 

 majority of cases there seems to be but one terrace below the general 

 level of the area adjacent to the valley. 



North and east of Toronto, the Trent, the Moira, and a few smaller 

 streams, have in part cut new channels in Trenton limestones. The 

 channels, which average perhaps twenty-five feet in depth, are straight- 



*This may be true of Pigeon and Chemong lakes also, 

 tin one case 400 yards in width. 



