2 88 A. P. COLEMAN 



their upper surfaces, is found 4 or 5 feet above Lake Ontario, 

 and perhaps indicates that the upper 30 feet are of later age 

 than the Iowan. Farther west along the shore of the lake the 

 till sheet is not more than 10 or 15 feet thick. So far as known, 

 this layer of till is nowhere thick enough to have modified 

 greatly the shape of the surface, which may now be roughly 

 sketched. 



The highest point of the Hudson River shale near Toronto 

 is at Weston, 7 miles northwest of the city, where it reaches 

 about 200 feet above Lake Ontario. In the Don valley the 

 shale runs from a little below lake level near the mouth of the 

 river to 30 feet above it 2 or 3 miles to the north ; but 5 miles 

 east of the Don a well sunk at the foot of Scarboro' Heights 

 failed to reach the rock at 41 feet below the lake, and beyond 

 this bedrock is not found for a long distance, first appearing at 

 Pickering, 15 miles to the east. This implies a width of about 

 25 miles for the mouth of the valley occupied by the preglacial 

 (and also interglacial) Laurentian river whose old channel from 

 Georgian Bay to Scarboro' has been indicated by Dr. Spencer. 1 

 The valley narrowed somewhat rapidly, however, toward the 

 northwest, for on the upper reaches of the River Rouge between 

 Unionville and Markham numerous angular slabs of Hudson 

 River rock, evidently not far from their source, are found more 

 than 300 feet above Ontario. The distance from this point to 

 Weston is 16 miles in a southwesterly direction, and between the 

 two points, the ravines of the Don and wells which have been 

 sunk prove that a channel of considerable depth existed. 



1 Duration of Niagara Falls and History of the Great Lakes, pp. 18 and 19. 



Dr. Spencer's idea of a present channel 474 feet deep, running from Scarboro' 

 Heights across Ontario to the deep water near the south shore seems founded on an 

 erroneous sounding as marked on Bayfield's chart. A series of soundings carried out 

 by the present writer in 1898 across the supposed deep channel showed no such inter- 

 ruption in the gentle southward slope of the bottom, the depth at about the position 

 of the 474 feet sounding on the chart being 175 feet. Probably the 4 is in mistake 

 for 1, and the true sounding was 174 feet. The old channel across Lake Ontario was 

 filled in completely, so far as one can ascertain, by stratified clay and till in later 

 times and so has long ago disappeared. The fact that Scarboro' Heights, rising 375 

 feet above the lake, have been piled up since then may be considered sufficient proof 

 of this. 



