256 The American Geologist. October. 1902. 
James Geikie, ''' and others. A magnificent example ot 
a glacier excavated basin, though of comparatively small size, 
is that of Loch Lomond whose length is twert^'-five miles and 
greatest width, about seven miles, with its surface twenty feet 
above the sea. It varies in depth quite uniformly from a few 
feet over its broader extremity, to three hundrd feet in its nar- 
rower portion toward the opposite extremity.! It is said 
that all the existing great lakes between the United States 
and Canada, and perhaps those of northwestern Canada are 
bordered by deposits which indicate their former extension. 
The largest of all the glacial lakes in America has been called 
lake Agassiz. It occupied the basin of the Red River of the 
North and lake Winnipeg, having a computed area of about 
110,000 square miles. $ 
In some cases the advance of the ice sheet has marked its 
own limits in a very definite way by the rigid accumulation 
of material at it? edges known as moraines. These are espec- 
ially prominent in connection with later advances of the ice, 
and traverse the interior in America, in looped courses, such 
that the term interlobate moraines has been given to parts of 
them. 
It is evident that this great continental glacier in its action 
upon the more elevated portions of the area over which it ex- 
tended reduced such portions and transferred the detritus to 
lower levels. When the ice melted the accumulation of earth 
material, in what is designated as till, boulder-clay, ground mor- 
aine, temiinal moraine, et cetera, and which has been instru- 
mental in producing smoothed and striated surfaces, was left 
as the valuable witness of the ice work. This material known 
as a whole as "glacial drift," was deposited very unevenly and 
irregularly over the glaciated area. 
The northern central states ofifer a peculiarly interesting 
and instructive field for a careful study of this subject. They 
embrace areas overspread wnth glacial drift, as well as the drift- 
less territory, which is perfectly free from such material. This 
latter non-glaciated area of about 10,000 square miles in Wis- 
consin and adjoining states is most remarkable because it stands 
in the midst of a vast tract overspread with drift on all sides, 
•Great Ice Age. 
f Geikie, Great Ice Age (Appendix), 
vey, 1896. 
J See 'Glacial Lake Agassiz,' Upham, Monograph 25, U. S. Geol. Sur- 
