Glacier Work.— Scott. 255 
it is seen that the pressure exerted near the bottom is verv great 
and an abrupt reverse of the general slope would readily pro- 
duce, as noted for the Alpine glaciers, a movement toward the 
surface ; in consequence, the boulders are moved upward into 
the zone of fracture, and in the specific cases under consider- 
ation doubtless the ice in the upper zone would be ab- 
normally wrenched and shattered so that its den- 
sity as a whole is much decreased. The bould- 
er may thus be stranded upon the top of the obstacle 
opposing the glacier motion, and by the ab'sorption of the sun's 
heat to a greater or less extent which would still further de- 
crease the effect of the energy of the ice immediatelv around 
it, retain. its position after the ice had finally disappeared. It 
is therefore quite conceivable that a boulder might start from 
the neider portion of a glacier and ultimately be found even 
hundreds of feet above its source. Frec[uently the boulders of 
this nature have a non-glacial aspect which is quite natural 
since they may have travelled i-n the ice mass without suffering- 
any abrasion whatever. The point is well illustrated by the 
knapsack which was lost in a crevasse on the Telefre glacier 
on July 29th, 1836, and was disgorged by a coalescent glacier 
ten years later after having travelled in the ice a distance of 
over 4300 feet. * 
The occurrence of immense erratics in America does not ap- 
pear to be so prominent as in Europe, .though several examples 
are known. The minor ones, however, have been very signifi- 
cant in determining the extent of drift over important areas, t 
Another effect of both erosion and transportation is found 
over the tracts of retired ice sheets in the abundance of glacial 
lakes. Some of these clearly were formed by erosion as they 
lie in ice worn rock basins. Again where glacial detritus is 
prominent they rest in depressions formed by this material and 
their origin cannot be assigned to any influence other than that 
of the ice, either directly or indirectly. The character of the 
climate and natural conditions of topography influenced to an 
important extent the formation of such lakes. Northern Eu- 
rope abounds with these lakes, the particular conditions as to 
the formation of which are well shown bv Archibald Geikie, t 
*E(]in. Phil. Jour. 1S47. 
tPrelim. Paper on DriftlfFs Area of the Upper Mississippi Valley. 
C. Chambcrlin and R. D. Salisbury, 1886. 
JText-book of Geology. 
