GLACIAL AND INTERGLACIAL BEDS 289 



We may suppose then that the great preglacial valley, though 

 coated with a sheet of bowlder clay by the Iowan ice, had 

 probably much the same form and dimensions at the end of that 

 ice invasion as before. If there was an interglacial episode 

 similar to the postglacial lake Iroquois no certain remains of its 

 beach deposits are known, and the level of the water when 

 the laying down of the Toronto Formation began was not 

 greater than that of Lake Ontario and may have been consider- 

 ably beneath it ; for the lowest unio beds lie more than 40 feet 

 beneath the present lake at Scarboro'. 



Before the formation of similar beds at higher levels consider- 

 able erosion took place, as at the bend of the Don where these 

 deposits occupy an old river channel cut through the Iowan till 

 into the Hudson River shale to the depth of at least 16 feet, as 

 shown by a cutting of the Don, at this point 19 feet above Ontario. 

 The bowlder clay has been cut through to the shale in the western 

 part of Toronto also, as shown in a well bored for purposes of 

 exploration, the bottom of the interglacial deposits being 17 feet 

 above Lake Ontario. It seems clear that rivers had been at 

 work for some time before the unio beds were formed. 



WARM CLIMATE BEDS OF THE DON VALLEY 



The earliest beds of the Toronto Formation were deposited 

 on the eroded surface of the Iowan till or on the shales which 

 had been laid bare beneath it by river action ; and they were 

 formed probably in the shallow waters of a lake, though some 

 features suggest the action of currents. At the bend of the 

 Don, coarse, little rounded shingle of the harder layers of the 

 underlying Hudson River rocks makes the lowest bed visible 

 above the present river, and suggests the action of a current 

 rather than of waves. Thick sheets of vegetable matter, 

 greatly decayed twigs, leaves, reeds, etc., with trunks and 

 branches of trees are interbedded with the shingle, however, 

 showing that the current could not have been swift. Possibly 

 these beds were formed just at the mouth of a small river like 

 the present Don, where it entered a lake standing 20 or 30 



