GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 95 



the basin, the amount of which will differ with the local conditions. In this way 

 we may account for the delay in the arrival of the crest at the neVe" line in the 

 years 1897-9. 



9. FORMER ACTIVITY. 



a. Rock scorings. The former work of the glacier is shown in great 

 beauty and variety upon the mass of bedrock now being gradually uncovered 

 near the nose. The hard rock features of Pleistocene glaciation are all here for 

 study by those interested, many of them indicating the direction of ice move- 

 ment and hence of practical value in the field. 1 Excellent examples of the so- 

 called roches moutonnees occur, groups of which in the distance often resemble 

 crouching sheep (plate xxxiv, figure 3). In the specimen figured, the ice moved 

 from right to left across this projection of bedrock, the up-stream, or stoss side being 

 rounded and smoothed, while the down-stream, or lee side, was affected slightly, 

 or not at all. Portions of the rock were polished, as the ice was rubbed vigorously 

 across, and where the ice held rock fragments against it, systems of approximately 

 parallel scratches were produced, some so fine that they must have been made by 

 sand grains. At the last stage of the disappearance of the ice from this particular 

 roche moutonne'e, a small clump of rock fragments was gently dropped upon the 

 upper surface in insecure position. An inspection of this and the adjoining rock 

 in the figure, shows a system of parallel joints, dipping down-stream at a steep 

 angle. From the lee side of the central roche moutonne'e it is apparent that an 

 entire block was pried loose by the ice and that a little more vigorous action at the 

 joint, just beginning to open, would have removed bodily nearly the entire block. 

 This action is known as "plucking," already described in connection with the 

 Yoho (page 79), by which the work done in a few days may exceed the erosion 

 of years. Places may be seen upon the surface where a rock engaged in produc- 

 ing a shallow groove has made a succession of jumps and given rise to a series 

 of short parallel curves, more or less closely placed, with their concavities 

 directed down-stream. These are the "chatter-marks," the production of 

 which may be illustrated by pushing a dry finger over a polished surface. In 

 other cases rocks embedded in the under side of the ice have been suddenly 

 brought into action, producing a crescentic gouge, with its convexity directed 

 in the direction of flow. 2 The bedrock here being a schistose conglomerate 

 with rather coarse, hard masses embedded in a softer matrix, there have been 

 produced the "knob and tail," or "knob and trail" phenomena, so useful often 

 in determining the direction of ice flow in the case of Pleistocene glaciers. In 

 one case examined there appeared a dark colored knob of harder material, 

 which the ice was unable to cut away as rapidly as the surrounding schist. 

 The projecting knob had partially protected the softer material in its lee, 



1 A most valuable paper by Chamberlin upon the effect of ice upon rock will be found in the Seventh 

 Annual Report of the Director of the U. S. Geol. Surv., 1888, page 155. 



2 See paper by Gilbert read before the Cordilleran Section of the Geological Society of America at its 

 1905 winter meeting. " Crescentic Gouges on Glaciated Surfaces," Bulletin Geol. Soc. of Amer., vol. 17, 

 PP- 303-3 U. 



