184 Part II. Chapter 2; 
Saskatchewan gravels laid down by the waters derived from the ice-front. The 
lowest drift deposits of the Mississippi Valley are provisionally regarded as 
equivalents of the Albertan drifl. A great retreat of the ice, if not its entire 
disappearance brought about interglacial conditions at least in the Mississippi 
Valley (Aftonian stage). The land area exposed by the retiring ice was 
rapidly reclothed with vegetation which in many places in Iowa, Minnesota, etc., 
formed accumulations of peat, sometimes to the depth of 25 feet. The Kansas 
stage represents the greatest extension southwestward of the ice-sheet, when the 
glacier descended again from the north nearly to the mouth of the Ohio River 
and spread across Iowa and Missouri far into Kansas. Eastward, the ice-sheet 
extended across the Mississippi River into Illinois. Again came a time of 
retreat, when Kansan till was eroded, soil was formed and peat deposited 
upon it, derived from vegetation which a second time had migrated into the 
barren area (Buchanan stage). A renewed extension of the ice laid down 
the Illinois till-sheet. A fourth recrudescence of the glacier (Iowan stage) 
occasioned the deposit of another till-sheet of an extent not yet determined, 
which is best displayed in northeastern Iowa. This stage was followed by 
interglacial deposits which are perhaps contemperaneous with those which 
are so well shown near Toronto along:the valley of the Don and at Scar- 
borough Heights and elsewhere in Canada and are a possible equivalent of 
GEIKIE’s Neudeckian of the old world. The beds form a succession of fine 
shales and sandstones, that lie between two sheets of glacial drift and contain 
fossil plants and animals forming a Pleistocene fossil flora and fauna '). 
2. The Pleistocene Flora. 
With the exception of Acer pleistocenium, it is a noteworthy fact that all 
the plants of the Pleistocene flora were such as are now represented in the. 
same localities, or, in the case of the Don Valley, by plants which find their 
most northern distribution at or near that region, and the somewhat unequal 
distribution thus indicated at once suggests definite climatic changes during 
Pleistocene time, as represented by the northern and southern migration of 
particular types of plants. The definite and abundant occurrence of Maclura 
aurantiaca (= Toxylon pomiferum), Funiperus virginiana, Quercus obtusiloba, 
O. oblongifolia, Asimina triloba, Chamaecyparis sphaeroidea (= Cupressus 
thyoides) and Fraxinus quadrangulata points with out question to the pre- 
valence of a much warmer climate than now prevails, while on the other hand, 
the equally abundant occurrence of the boreal types at Scarborough points to 
the existence of a colder climate at the time these deposits were laid down. 
It is, therefore, clear that in the region of Toronto during Pleistocene times 
1) PENHALLow, D. P.: Contributions to the pleistocene Flora of Canada. Transactions Royal 
Society of Canada, second ‘ser. 1896—97. II. sect. IV: 59—77; do with a Committee, Canadian 
pleistocene Flora and Fauna British Association Advancement Science 1898: 522—529; DAWSON, 
J- W., and Committee, Canadian pleistocene Flora and Fauna British Association 1900: I—12. 
