586 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



is practically unique in the whole Boundary section, recalling a type of morainal 

 landscape richly illustrated in Wisconsin, New York, Massachusetts, and else- 

 where. The glacial deposits flooring the trench are chiefly sandy till often 

 veneered with washed gravels and sands. The till has been subglacially moulded 

 into hills of drumloidal form, some of which have the characteristic, smooth 

 profiles of typical drumlins. (Plate 53, Fig. B.) A score of such lenticular 

 hills are mapped within the five-mile belt. Their longer axes are regularly 

 directed down the trench, a little east of south. The thickness of the drift 

 could not be determined, but it must in places amount to several hundred feet 

 and may average 200 feet or more. 



Between the drumloidal hills are numerous pits or kettle-holes, some of 

 which are well over 100 feet in depth; a few hold small lakes. These depres- 

 sions seem to be of origin quite similar to those occurring in the drumlin 

 areas of the eastern United States and of Europe. Some appear to represent the 

 hollows once occupied by blocks of stagnant ice (true kettles). Others were 

 due to the inevitable inequalities of the subglacial deposition of drift. 



Besides the till deposits, veneering kames and sandy plains of plainly 

 water-laid material were observed. Like the drumlins their surfaces have been 

 very little affected by post-Glacial erosion. Occasionally Glacial stream- 

 channels are incised in the drift. In one ease the channel ends suddenly in a 

 large kettle-hole, the floor of which lies thirty feet below the bottom level of 

 the channel. 



At the eastern edge of Tobacco Plains the drift deposits have been eroded 

 to a depth of from 200 to 300 feet, to form a flat-floored channel about 800 

 yards in width. This channel is said by the settlers to extend as far north as 

 the Elk river and, according to them, represents a former bed of that stream. 

 The channel fades out on the lower ground a few miles south of the Boundary 

 Line. Its origin was not finally worked out. Fairly large alluvial fans have 

 been built out on the floor of the channel, showing considerable antiquity; it 

 may have been excavated in late Glacial times. 



Purcell Mountain System. 



For the Purcell system the upper limit of the ice was rather definitely fixed 

 on the high ridge running south from the Boundary Line just east of the 118th 

 meridian. As in the Galton range the limit is practically at the 7,300-foot 

 contour. The highest summit bearing actually observed striae is 7,100 feet in 

 elevation. The direction of average movement across the ridge-tops was S.S.W. 

 Strong deflections were, however, observed at many elevated points where local 

 topography controlled the directions of the ice-currents. In the lower levels 

 the ice was similarly controlled by bed-rock relief. At all times the flow was 

 southward along the depressions for the ice which filled the Bocky Mountain 

 Trench, the Yahk river valley, the Moyie river valley, and the Purcell Trench. 

 These great troughs naturally controlled the drainage of the ice flood. 



The depth of the ice over the Yahk river valley must have been about 

 4,000 feet; over the Moyie, 4,500 to 4,600 feet; over the Purcell Trench, about 



