3i 



including 42 trees and other flowering plants with several 

 mosses, 41 shell-fish, 72 insects, and 5 or 6 mammals — about 

 165 or 170 species all told. 



outline; of climatic and physical changes. 



The Toronto interglacial period included great changes 

 in climate and in physiographic features. During the re- 

 treat of the first ice sheet no doubt the climate slowly 

 changed from Arctic to subarctic and finally to temperate, 

 and probably the valley was at first occupied by a great 

 glacial lake when thawing had proceeded so far as to free 

 the basin, but not its outlet toward the northeast. These 

 earlier stages of the interglacial time have left no visible 

 record, though they must have required thousands of years 

 to accomplish. 



The first episode in the Don beds shows a river flowing 

 into a lake lower than the present, with a rich deciduous 

 forest on its shores. There followed a rise of water in the 

 lake, probably by the upwarping of its outlet, to 60 feet 

 above the present level. This time of warm climate lasted 

 long enough for the deposit of 45 feet of sand and clay in 

 a delta several square miles in area, and for the growth of 

 generations of forest trees. 



At length there came a rise of the waters to 150 feet 

 or more above the present lake, when delta beds were laid 

 down covering more than 100 square miles. At one point 

 there are 672 annual layers in less than 20 feet, so that 

 the whole thickness must have required some thousands of 

 years to deposit. The climate had become colder, as shown 

 by the plants and insects, and was like that of northern 

 Ontario at present. 



Next, the great lake was drained to a level 16 feet below 

 lake Ontario, and three river valleys were carved in the 

 delta, a wide one toward the west at the present site of 

 Toronto, a narrower one at the Dutch Church and another 

 wide one towards Highland creek. These valleys had gently 

 sloping sides and were much more mature than the present 

 valleys of the Don and the Humber. To cut them the rivers 

 must have required several thousand years. 



Finally, Arctic conditions came on and the ice advanced 

 once more from the northeast, covering the eroded surface 

 of the region with a second sheet of boulder clay. The 

 climatic cycle was complete. 



