CEXOzoic (interior). 295 
it is parallel — and extends northwestward towards its head waters, 
and crosses into Canada at the east foot of the Rocky Mountains. 
There is also a southern drift (Orange Sand, etc.), in the val- 
ley of the lower Mississippi and in the Gulf States, whose re- 
lation to that of the north is not yet fully explained. 
In the interior of the Continent, the Plistoceue deposits often 
rest upon glaciated rock surfaces, which have not been defaced 
by weathering. These are best developed in the lake region or 
over areas radiating therefrom. The general direction of the 
striation is to the southward, radiating from the basin of the 
Great Lakes, (whose greatest depths are mostly from 750 to 1026 
feet). From the valley of the Ottawa River to that of the St. 
Louis — southwest of Lake Superior — the direction of the striae 
is traceable across the deep lake basins, over the highlands — 
separating the lakes from the arctic drainage (at no greater alti- 
tudes than the more southern hills) — to a common centre in the 
great broad open basin of Hudson's Bay, 1600 feet or more below 
the glaciated hills to the south. The same phenonaena hold true 
for northwestern Canada, where the movement was from Hudson's 
Bay, or the more southern low plains, toward the higher Coteau 
de Missouri and higher plateaus, at the foot of the Rocky Moun- 
tains. 
The deposits may form vast irregular sheets of clay, sand, 
gravel or boulders; may assume the form of great ridges of the 
same materials, having a thickness of even 500-700 feet (exclu- 
sive of that filling buried valleys) ; or may be in regularly stra- 
tified beds. Some of the Plistocene deposits are of wide extent. 
The positions of others more or less disconnected, are often difficult 
of correlation, on account of their changing characteristics, or 
our imperfect knowledge of them. This difficulty is increased 
owing to a variety of results being produced simultaneously by 
different agents ; and even here geologists are not agreed as to 
the causes. Consequently only the more important series of the 
Plistocene system are provisionally given. In ascending order: 
6. Terrace and Beach. 
5. Saugeen clay. 
4. Erie clay. 
3. Upper Drift or Till. 
2. Vegetal bed. 
1. Lower Drift or Till. 
