582 O. H. HERSHEY TERTIARY AND QUATERNARY GEOLOGY 



Eiver, in cutting down into the gravel, encountered a buried rock ridge 

 near the south side of the valley, and at Spokane Falls it has not yet 

 cut its canyon through a buried basalt ridge. The size of the canyon 

 below the falls indicates that the gravel filling of the valley is Wisconsin 

 in age. Hay den Lake also occupies a mountain valley seemingly dammed 

 by this geologically very young gravel deposit. 



The youth of Coeur d'Alene Lake is further testified to by the fact 

 that the streams flowing into it have failed to fill its basin with sedi- 

 ment. The Coeur d'Alene Eiver has built a delta from the original 

 head of the lake near Cataldo, 25 miles, to near Harrison". This con- 

 sists largely of natural dikes bordering the deep, narrow channel of the 

 river. Behind the dikes are marshes and lakes, including Eose, Killar- 

 ney, Hidden, Medicine, Cave, Swan, Black, Blue, Thompson, and Ander- 

 son lakes. The most easterly of these lakes occupies a valley of con- 

 siderable extent eroded after the distribution of the boulders marking 

 the earliest glacial stage. 



Valleys of Clearwater Country, Idaho 



In the region of the South Fork of the Clearwater Eiver, in Idaho 

 County, Idaho, most of the higher mountains are long, relatively even- 

 crested, moderatel}'' smooth ridges that may represent an ancient pene- 

 plain.^ The highest peaks, such as Pilot Knob (altitude, 7,160 feet), 

 Buffalo Hump (altitude, 8,320 feet), Anderson Butte (altitude, 6,800 

 feet), Gospel Mountain, and Umbrella Butte, seem to rise above the level 

 of the long, smooth ridges and may represent monadnocks on the pene- 

 plain. The main streams eroded broad, basin-like valleys to a depth of 

 several thousand feet below the old peneplain level, and floored them by 

 wide and in places thick deposits of gold-bearing river gravels. The 

 South Fork of the Clearwater Eiver, where I saw it in the vicinity of 

 Newsome and Ten-mile creeks, is entrenched in a narrow, rocky canyon 

 which has been cut l)eneath the gravel-floored old valley to a depth of 

 probably 800 to 1,000 feet. It resembles the Pleistocene canyons of the 

 Sierra Nevada region and is probably of the same age. 



The river passes out of the mountain region into that of the "prairies," 

 or plateaus, which are portions of a great, gently undulating basalt plain 

 that merges with the great lava plain of the eastern Washington wheat 

 region. T did not follow the vallev of the South Fork of the Clear- 

 water so as to determine whether the floor of the broad upper valley 



^ r.indfivon also oiUiM-tained this view. See I'rofessional Paper No. 27, U. S. Geol. Sur- 

 vey Pub., 1904. 



