338 ON DRIFT ICE AS AN ERODING 
25 feet, sweeping everything clean before them. The result is, 
that in very few places in Labrador are there any boulder ridg3s 
such as we see in Nova Seotia. Therefore, these latter can 
hardly have been the product of exactly the same conditions as 
are present in Labrador. 
Ice action on ridges, shoals and low points, consists of an 
onward rush of ice as described above, but the ice in front is left 
poised on the ridge until pushed forward by other ice masses 
brought in by succeeding waves. Loose stones moved on those 
nearly flat or slightly sloping surfaces are nearly always rolled, 
and not pushed. But any stones caught beneath the ice, act as 
gravers and score the rock over which they slide. Strize made 
by stones in such a position are easily distinguished from the 
straight scratches left by retiring ice on a sloping shore, and 
also from striz of acknowledged glacial origin. Such striz are 
sometimes curved owing to the swinging of the ice mass and 
consequent change of course of the graver beneath. But they 
often form a furrow of which different parts run in different 
directions. This latter is owing to irregular and repeated pushes 
from ice in the rear. 
Another form of marking is made when a large quartz or 
vranite boulder is rolled on a soft slate bottom. It then produces. 
a series of notches and irregular scratches. 
But it must not be forgotten that the first ice thrown up in 
the spring usually cleans off the debris previously gathered, 
after which it 1s exposed to months of continuous wear by ice, 
sand, and water. And should such striz be covered by a layer 
of debris, this is certain to be worked over to a great extent 
before being swept into deep water, or thrown beyond the reach 
of the highest tides. Therefore, the preservation of stric in 
such positions seems to be well nigh impossible, except on the 
inner side of the ridge or point, where its formation is extremely 
doubtful. 
There are, however, two classes of marine strive which are 
not usually polished out. 
