186 G. F. Wright — Continuity of the Glacial Period. 



action are not only the frequency of Glacial rock scorings to 

 the north, but, among other things, the knobs and depressions 

 in the moraine and in the head of the moraine terraces. 



So far as the rock scoring and analogous phenomena are con- 

 cerned, it is a matter of course that the agencies producing 

 them increased in efficiency as you recede from the border ; for 

 all the ice which reached the border moved over the central area 

 of glaciation together with all the ice that reached to any distance 

 beyond the central area. The cup and saucer character of the 

 deposits at the edge of the glacier where a moraine was formed, 

 is probably due in considerable measure to the irregular melting 

 away of ice which originally underlaid the deposit. The 

 present level of the deposits is not by any means always 

 the original level. At Ackley and for a short distance in the 

 valley below, for instance, the ice was very likely originally at 

 one time one or two thousand feet thick, and resting upon the 

 rock bottom of the river trough. In the process of ablation, 

 when the ice at the front had wasted to a thickness of 200 or 

 300 feet more or less, it was probably covered, as the front of 

 the Malaspina and a portion of that of the Muir glacier are, 

 with a vast amount of pebbles, sand, gravel, and clay which had 

 gradually worked down upon it. In the place where the under- 

 lying ice was thickest, its melting away would let the superin- 

 cumbent mass down to a lower level than that which rested 

 upon a more permanent basis 



This seems to be just what we have at Ackley. A terrace 

 upon the east side two miles below Ackley, rises to a height of 

 150 feet above the flood-plain, and is level topped. On follow- 

 ing it up the river, however, it soon ceases to be level topped, 

 and exhibits knolls and saucer-shaped depressions character- 

 istic of true morainic accumulations. At the same time it grad- 

 ually falls to a lower level, so that at Ackley the moraine is not 

 over forty or fifty feet above the flood plain, and north of 

 Ackley these deposits in the valley give place to a long swampy 

 stretch of country extending for several miles up the stream, 

 and which is absolutely devoid of surface gravel. The expla- 

 nation of this is that north of the portion of the terrace first 

 mentioned the underlying ice in the valley was assuming greater 

 and greater importance until above Ackley it reigned supreme. 

 When at length it melted away, the gravel gradually and 

 irregularly settled down to lower levels as the ice was thicker 

 and thicker, until back of the moraine there was nothing but 

 space left. If I mistake not, I have seen a similar process 

 going on, on the east side of the Muir inlet, where the front of 

 the ice rests upon the main land. If instead of debouching at 

 its center into the tide water, this glacier was stretching out into 

 a valley above tide with a distinct trough in the center, we 



