60 Professor A. P. Coleman — Changes of Level. 



Penballow, an assemblage of forest trees like that of Oaio or 

 Pennsylvania, some of them reaching their northern limit to the 

 south of Toronto, others still growing in southern Ontario. One 

 of them, Acer pleistocenicum, is extinct. Tree trunks 18 inches 

 across are found at various levels in the Don beds, generally too 

 much flattened by the pressure of the later ice-sheet to permit of 

 counting their annual rings, though one that was better preserved 

 than usual was found to have 120 rings in a width of 4 inches, 

 and must have been 7 inches in radius when complete, i.e. 200 years 

 old. These trees occur at the very bottom of the beds as well as 

 at several levels above, and must represent different generations of 

 forest. The upper or Scarboro' beds contain trees and other plants 

 as well as beetles, of a cool temperate climate, like that of the north 

 shore of Lake Superior, according to Dr. Macoun and Dr. Scudder. 

 There are no Arctic nor sub- Arctic forms among them, and no glaciated 

 pebbles nor ice-transported boulders; and the stratified clay is 

 more ferruginous and less calcareous than any of our stratified 

 glacial clays, burning to red brick instead of grey brick like the 

 latter. The large river which formed the delta must have drained 

 the upper lakes by a direct channel from Georgian Bay on Lake 

 Huron to Scarboro' on Lake Ontario, and could not have drawn its 

 waters from glacial sources. 



It may be noted that at the very beginning of these Interglacial 

 beds trees, such as three species of oak, an elm, the red cedar, and 

 the pawpaw, were growing in the Don valley, showing that the 

 climate was already warm, the last tree indicating a dry and hot 

 Summer like that of Ohio. The isotherms evidently ran 150 miles 

 north of their present position in eastern America. 



Let us now turn to another side of the evidence. At the beginning 

 of the deposits the water in the Ontario basin stood no higher than 

 at present, perhaps lower, for streams eroded their channels through 

 the Boulder-clay, and even 16 feet into the solid rock beneath, as 

 shown in one of the Don sections. Afterwards the water in the 

 basin rose to 60 feet above Lake Ontario at the end of the Don beds, 

 and to 152 feet or more above that level before the close of the 

 Scarboro' beds. There is no doubt about this point, for the deposits 

 may be followed for miles with fine and even stratification, and 

 oould not have been formed by glacial rivers on a sloping land 

 surface, as suggested by Mr. Warren Upham.^ 



Afterwards there was a fall in the level of the water to a point 

 much below that of Ontario, and rivers had time to cut valleys 

 more than 150 feet deep and from a mile to five miles in width 

 through the Interglacial clay and sand. We have, therefore, an 

 interval of erosion with low water after the retreat of the earlier 

 ice-sheet before the warm-climate beds were laid down ; and a very 

 long stage of erosion with still lower water after the cold-temperate- 

 climate beds were deposited ; while there was a stage of high water 

 between. How shall we account for these changes in the Interglacial 

 water-levels ? 



^ Amer. GeoL, vol. xxviii, No. 5, p. 315. 



