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 ridges 

 such as we see in Nova Scotia. 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. Striae 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 striae of acknowledged glacial origin. Such striae 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 

 granite 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 is exposed to months of continuous wear by ice, 

 sand, and water. And should such striae 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 striae 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 striae which are 

 not usually polished out. 



