REPORT OF TEE CHIEF ASTRONOMER 587 



SESSIONAL PAPER No. 25a 



5,500 feet. The average depth for the whole Purcell system at the Forty-ninth 

 Parallel was about 2,500 feet. At the summits of the Yahk and McGillivray 

 ranges, a few small nunataks projected a few hundred feet. Elsewhere the 

 whole mountain system, including ninety-nine per cent of the Boundary belt, 

 was completely smothered under the ice. This fact doubtless partly explains 

 the relative rarity of cirques in these mountains (about a dozen in the Boun- 

 dary belt). The relief was not sufficient to generate valley ice-sheets which 

 could endure long enough for the quarrying out of many amphitheatres. The 

 glacial erosion of the Purcells was thus chiefly accomplished under the all- 

 mantling ice-cap and not at the head-walls of local glaciers. 



Five or six of the cirques observed in the Boundary belt have been opened 

 on the basset edges (those facing the direction of dip) of the strata. The head- 

 wall of each cirque has been driven into the mountain against those edges. 

 The relation shows vividly the contrast in the essential processes of normal 

 subaerial erosion when compared with the process of cirque development. 



Throughout the Boundary belt drift deposits may at intervals be found 

 in the Purcells, but they are not so heavy as in the more westerly ranges. The 

 deposits are naturally irregular and do not declare themselves readily as belong- 

 ing to definite or recognized types. It was noted that the slopes on each side 

 of the Yahk river, up to a level about 200 feet above it, have been washed very 

 clean of gravels and other drift material. The explanation is sought in the 

 hypothesis that toward the close of the Glacial period, this valley was occupied 

 by a very large and powerful river which was fed by the rush of waters from 

 the melting ice-cap farther north. This temporary river must have been over 

 a mile wide and at least 200 feet deep in the middle part. Its point or points 

 of origin and its course outside the Boundary belt were not determined. Wa 

 have here a type of many such problems in the nature and effects of late- 

 Glacial drainage of the Cordillera. Many seasons of special field work aided by 

 extensive, accurate mapping, will be necessary before this chapter in geological 

 history can be written. 



Between Porthill and McKim Cliff, a distance of four miles, the Purcell 

 Trench is floored with a thick mass of obscurely stratified clay, which contains 

 a few scattered drift boulders. The clay is of varying thickness and fills depres- 

 sions in the rock bench which outcrops at intervals through the same width of 

 the trench. Some patches of true boulder-clay and of washed gravels intervene 

 between the stratified clay and bed-rock. The gravels and boulder-clay have the 

 properties of the usual Glacial deposits. The massive stratified clay is fine- 

 textured and very homogeneous. It extends from Goat river six miles north 

 of the Boundary to some undetermined point south of Copeland, Idaho. As 

 shown on the map, the surface of the bench is not flat but varies from 2,000 

 feet or less to 2,300 feet in elevation. 



From the fact that the properties of the clay are sensibly like those of the 

 Kootenay river delta which is to-day growing out into the lake below Creston, 

 the writer is inclined to the view that the stratified clay of the Porthill bench 

 was laid down in a temporary lake. If this be true the western half of the 

 trench between Creston, Porthill, and points farther south, must have been 



