ICE-PERIOD IN AMERICA. 
85 
rock on the prairies of Illinois or Iowa. One 
may follow these boulders to the fortieth de¬ 
gree of latitude, beyond which they become 
more and more rare, while the finer drift alone 
extends farther south. 
It is not only, however, by tracking the 
boulders back to their origin in the North that 
we ascertain the starting-point of the whole 
mass; we have another kind of evidence to 
this effect, already alluded to in the descrip¬ 
tion of the roches moutonnees. Wherever the 
natural surface of any hill, having a steep 
southern slope, is exposed, the marks are al¬ 
ways found to be very distinct on the north¬ 
ern side and entirely wanting on the southern 
one, showing that, as in the case of many of 
the roches moutonnees in Switzerland, the 
mass moved up the northern slope, forcing its 
way against it, grinding and furrowing the 
northern face of the hill as it moved over it, 
but bridging the opposite side in its descent 
without coming into contact with it.* This 
is true, not only of bills, but of much slighter 
obstacles which presented themselves in the 
* Fuller description of these polished hills may be found 
in my work on Lake Superior. 
