EAtltiY GLACIATtON IN NORTHERN IDAHO 531 



d'Alene Lake, beyond the main basalt area. The lake must have existed 

 for a very short time, for no extensive typical lacustrine deposits were 

 made in it. The deposits of local debris bearing granite boulders may 

 have been formed by shore ice or wave action along the steep slopes 

 during the rising and falling of the surface of the lake, but this wave 

 action was not continued long enough at one level to form a recognizable 

 beach. 



The most interesting feature of this glaciation is its relatively great 

 age. The best measure of this is the size of the rock canyon excavated 

 in Prichard slate by the river between Kellogg and the mouth of Big 

 Creek subsequent to the abandonment of the 200-foot terrace as the 

 valley floor. This may average 300 feet deep and 500 feet wide at the 

 bottom. Below Pine Creek the river has cut at least 125 feet deep 

 into hard Cataldo quartzite. If the bouldery deposit had been formed 

 after part of tliis rock-cutting had been accomplished, I hardly think it 

 would have been so completely removed that none would remain in the 

 canyon or in the wide valley below Kellogg. There are no granite 

 boulders on the 600-foot terrace. Unquestionably the boulder deposit 

 is very much older than any glacial phenomena recognized in the high 

 valleys of the Bitter Eoot Eange ; it is probably as old as any drift-sheet 

 in the Mississippi Basin. 



Origin and Age of Cceur d'Alene Lake 



Coeur d'Alene Lake is a long, narrow branching body of water occupy- 

 ing a steep-walled mountain valley which is the continuation of the new 

 or post-basalt valley of the Coeur d'Alene Eiver. Near the southern 

 end of the lake the basalt forms terraces between the lake and the higher 

 mountains of pre-Tertiary rocks. It is my impression that the lake has 

 been produced, as suggested by Ransome,* by the damming of an old 

 mountain river valley by the accumulation of glacial overwash gravels 

 derived from a glacier retreating northward along the Purcell trench. 

 In the city of Cceur d'Alene, at the north end of the lake, the gravel 

 plain is overlaid by a bed of brown sand which slopes toward the lake. 

 The Spokane Valley above Spokane is floored by this broad, nearly level 

 gravel plain; the gravel in places is rather coarse and contains many 

 small boulders. The Spokane Eiver flows in a shallow trench which in 

 many places is no wider than the stream, suggesting the Upper Missis- 

 sippi Eiver in central Minnesota, where it is flowing in a narrow trench 

 cut in gravel plains of Wisconsin age. At Post Falls the Spokane 



* Professional Paper No. 62, U. S. Geol. Survey Pub., p. 18. 



