296 R. BELL — GLACIAL PHENOMENA IN « \\\l>.\. 



parallel grooves on their leaving a slight depression on such slopes. The 

 force would evidently come from the direction from which the grooves radi- 

 ate. 



Perhaps the readiest means of ascertaining the direction of movement is 

 afforded by the crescent-shaped markings bo frequently to he Been on gla- 

 ciated surfaces, but which have nol received the consideration they deserve 

 in thi> connection. These markings follow each other at short intervals in 

 rowa parallel to the stria?, their convex sides being towards the quarter from 

 which the movement came These markings are generally from an inch to 

 sis inches in diameter. Wherever they occur they seem to indicate great 

 pressure and appear to have been caused by hard stones firmly held in the 

 lower surface of compact ice moving forward per aaltum, a< if they had 

 stopped at each interval and actually crushed into the rock-surface by the 

 stupendous weighl above, ami then to have been forced along again a short 

 distance when another Btop ami another bruise in the rock occurred. 



When unaltered strata lie at low angles upon a nucleus of crystalline 

 rocks, there is :l marked difference in the effects produced by the action of 

 the passing ice-sheel according as the latter moved from the overlapping 

 Btrata onto thesolid nucleus or off the latter against the upturned edges of the 

 stratified rock-. In the former case no valley- are formed, and there is 

 oothing in the topography to indicate the junction of the two formations; 

 hut in the latter, great erosion has always taken place and valleys and 

 basins are formed whose width depends largely on the angle of dip and the 

 softness of the strata which have been scooped out. The strata are pre- 

 sented in the mosi favorable attitude for abrasion, especially when they 

 have been cracked by transverse anticlinals. The wearing-down pro, 



Would go on till the resisting rock-front had attained a heighl and weight 

 sufficient to counterbalance those of the glacier. The excavating proa — 

 would be greatly aided by the tendency, which seem- to exist, of the rocky 

 del >ris to rise from the base over heights lying in front, in the direction of move- 

 ment. These excavations are now generally occupied by lake- or channels, 



or they form valleys of liver.-. The St. Laurence helow Quebec, the North 



channel of Lake Union, and the long sounds of the east Coast of Hudson's 



bay are cases in point. The last named lie between the mainland ami the 

 long chains of islands which run parallel to it. The islands are composed 

 stratified rocks, dipping westward into the sea and having steep bluffs 

 facing inland or directly opposite to the general westward course of the 

 drift along that coast. The basins of lakes Ontario, Erie, Huron, ami 

 Michigan, as well a- that of Georgian bay, were excavated in a similar 



manner. Further north we have other example- of ha-in.- of ero.-ioii in 



lakes Winnipeg, Winnipegosis, Manitoba, Athabasca, and in Great Slave 

 lake, not to mention innumerable -mailer on< b. 



