GLACIAL AND INTERGLACIAL BEDS 287 



the important changes of climate indicated. In the following 

 paper an attempt will be made to give a connected history of the 

 events which have occurred in the Ontario basin during the time 

 represented by the Toronto Formation and the sheets of till 

 below and above it, generally held to belong respectively to the 

 Iowan and Wisconsin ice advances. 



RETREAT OF THE IOWAN ICE SHEET 



The retreat of the Iowan ice was probably accompanied by 

 one or more lakes similar to those whose raised beaches, formed 

 during the retreat of the last ice sheet, are so well marked 

 around the present great lakes. Though no remnants of Iowan 

 beaches are known to exist, there is strong faunal evidence of at 

 least one glacially dammed Iowan lake. When the region was 

 ice covered, all aquatic life must have been destroyed, so that 

 any species occurring in interglacial beds must have migrated 

 into the region from river systems beyond the reach of the ice. 

 The unios which are so striking a feature of the lower beds of 

 the Toronto Formation are Mississippi forms. As there is no 

 more proof of direct connection between the Ontario basin and 

 tributaries of the Mississippi during interglacial times than now, 

 we may suppose that these shellfish entered the basin in a 

 round-about way by means of an interglacial upper lake, drain- 

 ing at first past Chicago into the Mississippi, but afterwards 

 finding an outlet by the Laurentian river into the Ontario valley 

 and thence into the gulf of St. Lawrence. 



After the Iowan glacier had retreated so far that lakes 

 dammed by it had been drained, the St. Lawrence system of 

 waters no doubt returned to much the same channels as before 

 the advance of the ice, since there is no evidence that any great 

 thickness of drift had been left to block the way. The lowest till 

 at Toronto is generally thin, running from a foot or more near the 

 bend of the Don at the paper mill to 8 or 9 feet at the Gerrard 

 street bridge. At one point in the west end of the city, how- 

 ever, 35 to 40 feet of till occur, but a well-defined "bowlder 

 pavement," with all the stones at the same level and striated on 



