310 A. P. COLEMAN 



somewhat strongly insisted on in this paper, since many geolo- 

 gists who have worked only in regions where the Pleistocene 

 deposits are relatively simple in structure and not of great thick- 

 ness are apt to underrate the importance of interglacial periods, 

 looking on them as short episodes of retreat and advance in the 

 history of a single Ice age. The evidence adduced here points 

 to completely distinct Ice ages, separated by thousands of years 

 of mild climate. It is not improbable that the present time is 

 merely another interglacial period. 



An interesting result of the action of rivers and ice is found 

 in the change of relief in the region since the Iowan ice departed. 

 The valley of the Laurentian river, then probably a hundred feet 

 or more below the present level of Lake Ontario, is now replaced 

 by Scarboro' Heights rising 350 feet above the lake and pre- 

 senting the highest cliffs on its whole shore. 



A summary of the best marked stages in the Pleistocene his- 

 tory of the region is given below, special reference being made 

 to climates and water levels. The latter are of course not abso- 

 lute levels but only relative, since the region as a whole prob- 

 ably underwent important elevations and depressions during 

 Pleistocene times. 



STAGES OF TORONTO PLEISTOCENE 



i. Retreat of the Iowan ice sheet. 



2. Interval of erosion with water probably lower than at present. 



3. Don stage, warm climate trees and Mississippi unios, water dammed by dif- 

 ferential elevation toward the northeast to 60 feet above the present lake. 



4. Scarboro' peaty clays, cold temperate climate, with trees and mosses 

 and 70 species of extinct beetles, formed as delta by Laurentian river in 

 interglacial Scarboro' bay. 



5. Scarboro' stratified sand with some trees and freshwater shells of cold 

 temperate climate, delta completed, lake stands 1 5 2 feet above the present. 



6. Water drawn off by lowering of outlet, subaerial erosion of previous beds, 

 and cutting of river valleys more than 150 feet deep. 



7. Advance of Wisconsin ice front raising the water to about 160 feet as 

 shown by stratified interglacial clay, retreat for 50 miles and re-advance, 

 followed by two later retreats and advances, the water finally rising 360 

 feet above the present lake. 



8. Final retreat of ice sheet, followed by water levels of lakes Warren and 

 Iroquois and a brief entry of the Gulf of St. Lawrence into the Ontario 

 basin, which, however remained fresh. a p COLEMAN 



