208 SURFACE GEOLOGY. 
to. These are the beds to which I have alluded as constitu- 
ting, in the opinion of some geologists, a great glacial mo- 
raine, but from the fact that they are locally stratified, and 
overlie the older blue clays, I have regarded them as trans- 
ported not by glaciers, but by icebergs. 
Possibly some part of this Drift material may have accu- 
mulated along the margin of the great glacier, moved by its 
agency; but in that case we should expect to find in it abun- 
dant fragments of the rocks which outcrop in the region 
under consideration, whereas I have rarely, if ever, seen in 
these Drift gravels any representatives of the rocks under- 
lying the south margin of the lake basin. 
By whatever agency transported, the Drift gravels have, 
like the boulders, for the most part come from some remote 
point at the North, and were once spread broadcast along the 
southern shore of the inland iceberg-bearing sea. 
In the retreat of the shore line during the contraction of 
the water surface down to its present area, every part of the 
slope of the southern shore between the present water sur- 
face and the highest lake level of former times, i.e. all 
within a vertical height of three hundred feet or more, must 
in turn have been submitted to the action of the shore waves, 
rain, and rivers, by which if, as is probable, the retrograde 
movement of the water line was slow, these loose materials 
would be rolled, ground, sorted, sifted, and shifted, so that 
comparatively little would. be left in its original bedding ; the 
fine materials, clay and sand, would be washed out and car- 
ried farther and still farther into the lake basin, and spread 
over the bottom, to form, in short, the upper sandy layers 
of the Drift. 
At certain points in its descent the water level seems to 
have been for a time stationary, and such points are marked 
by terraces and the long lines of ancient beaches which have 
been referred to. A similar "lake ridge" now borders the 
south shore of Lake Michigan, where it may be observed in 
the process of formation ; and this seems to be the legitimate 
