32 GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 



the decimal point one place to the left. The greatest movement was shown by 

 plate II, almost 20 inches, and this was the plate nearest the centre. From 

 this point the motion diminished very gradually toward the west, but less rapidly 

 eastward, toward the crest of the medial moraine, and then fell off very suddenly. 

 The plates in the crests of the two lateral moraines had moved up-stream, sup- 

 posedly from the settling of the debris due to ice melting beneath. Plate 3 

 seemed to be located upon a stagnant portion of ice, showing no movement for 

 either the cool or the warm period, and then could not be found in 1905, appar- 

 ently because of the side cutting of a surface stream. For the warm period, 

 Jul}' 20 to 29, the results are given in the next column for ready comparison. 

 The motion is seen to be greater, almost twice as great for plate 14 and almost 

 three times as great for plate 15. Both plates vipon the lateral moraines had 

 moved ahead, although that upon the right lateral had not yet regained its 

 original position. Plate 2 showed no movement whatever for the warm period, 

 and number 4 showed no increase of movement. In column 9 there is given the 

 average daily motion for these 20 days of midsummer, which may be regarded as 

 typical of the season. For this period the greatest forward movement was shown 

 by plate 13, having a daily average of 2.736 inches. It is located some 287 feet 

 east of the centre of the glacier, and, as previously pointed out, showed the 

 maximum ablation. Number 3 showed no movement whatever. On September 

 5, 1905, the plates were again located, all being readily found except plate 3, 

 and their distances from the original line determined. These results are listed 

 in column 10, from which have been calculated the average daily motion and the 

 motion for a year. In all cases there was a down-stream movement indicated, 

 although very slight for the two lateral moraines. The greatest movement was 

 shown by plate 11, the one nearest the centre of the glacier, amounting to a total 

 of 76.3 feet for the entire period of 423 days, or a daily average of 2.165 inches. 

 This represents a j^early motion of about 66 feet. The central position of the 

 plate of maximum movement for the year was to be expected from the very 

 straight course of this part of the glacier. Its daily motion for the year is about 

 81 per cent, of its midsummer motion, which means that for the greater part 

 of the year the movement is fairly uniform. The table suggests that during the 

 season of maximum motion there are cross-currents set up in the ice, and, with 

 reference to the body of the ice itself, not the bed, even back currents. During 

 the year, however, the impulse is steadily and regularly forward. Studies upon 

 the dirt bands of Forbes, to be later discussed, indicate that as we approach the 

 steeper ice slope opposite Lefroy, the motion is more rapid than that given in the 

 table. 



b. Frontal changes. The rate of melting about the nose and side of the glacier, 

 in connection with the rate of forward movement of the ice, determines the 

 behavior of the front. When these two factors are balanced the glacier appears 

 to halt, and, if carrying debris, begins to build a terminal moraine. If either 

 the rate of forward movement, or the rate^of recession from melting, is in 

 excess then the glacial extremity advances, or retreats, entirely regardless of the 



