REPORT OF THE CHIEF ASTRONOMER 579 



SESSIONAL PAPER No. 25a 



Clarke Eange. 



During the Glacial period, the Clarke range formed a strong 

 divide between two sets of local glaciers. The eastern set headed in a 

 multitude of cirques which were connected by a more or less continuous snow- 

 field covering the slopes just east of the present ' Great Divide.' Some of these 

 valley-glaciers descended to the wide trough now occupied by Waterton lake 

 where the various sheets coalesced with others from the Lewis range, to form 

 a broad north-flowing intermont glacier. Other glaciers moved down the Clarke 

 range canyons directly to the Great Plains and there, with the wide Waterton 

 glacier, formed part of the eastern piedmont ice-sheet of the Cordillera. 



The western set of local sheets in the Clarke range descended their high- 

 grade valleys and merged into the wide intermont glacier which largely filled 

 the valley of the existing North Fork of the Flathead river. (Plates 50 and 12). 

 This great sheet may be shortly referred to as the North Flathead glacier. 



The writer has made no special study of the areas occupied by the Water- 

 ton glacier and eastern piedmont glacier. It is of interest, therefore, to note 

 the conclusion of Calhoun that the Waterton, Belly River, and Lees Creek 

 glaciers were probably confluent in one large piedmont sheet. 



The main or upper Waterton lake doubtless owes its origin to the activities 

 of the Waterton glacier. How far the basin is due to glacial excavation and 

 how far to morainal damming cannot with certainty yet be declared. That the 

 pre-Glacial valley has been considerably widened and deepened through glacial 

 erosion is shown by the fact that Oil Creek ' hangs ' at least 200 feet above the 

 rock floor of Waterton lake. This break in the stream gradient is best explained 

 as due to the more rapid glacial erosion of the main valley-floor as compared 

 with the degradation of Oil Creek valley by its own, much smaller glacier. 

 The relation suggests that the main Waterton lake in large part occupies a true 

 rock-basin excavated by the powerful Waterton glacier. 



The original Waterton lake has been divided into three unequal parts; 

 the middle and lower lakes have been separated from each other and from the 

 main lake by the growth of the post-Glacial deltas of Pass creek, eastward from 

 the Clarke range, and of Coal creek westward from the Lewis range. 



The erosive power of the Pleistocene high-level valley glaciers of the Rocky 

 Mountains is wonderfully illustrated throughout the whole system from the 

 Missouri river to Yukon territory. Thousands of shallow pre-Glacial valleys 

 have been greatly deepened, their walls steepened, with the generation of 

 abundant U-shaped cross-sections. Thousands of the valley-heads have been 

 modified into typical cirques or amphitheatres, many of which are floored with 

 small rock-rimmed lakes or tarns, those ' gems of the mountains.' The finest 

 examples of these glacial effects occurring on and near the Forty-ninth Parallel 

 are to be found in the Clarke and Lewis ranges. Probably nowhere in North 

 America have cirques and glacial troughs been more tellingly mapped than 

 in the Chief Mountain quadrangle of the United States Geological Survey 

 Topographic Atlas. We owe this sheet to Matthes and Sargent, whose unusual 

 accuracy and rare artistic skill have portrayed the topography of some 800 



25a— vol. iii — 33 



