824 



INDEX TO THE STEATIGRAPHY OF NORTH AMERICA. 



K-L 11. SNAKE BIVEE, BASIN, OREGON AND IDAHO. 



The Payette formation of the Snake River valley is described by Lindgren^^ 

 as follows : 



During the earlier part of the Neocene (Miocene) a large fresh-water lake occupied at 

 least the lower part of the Snake River valley, and its sediments are now prominent features 

 of the region. For these lake beds the name Payette formation is proposed, and their age is 

 determined as upper Miocene. 



It extends over large areas to the north of the Payette, along the flood plains of the Snake 

 River, and is seen to occupy vast areas in Oregon between the mouth of the Owyhee River 

 and Weiser, where the Snake River canyon begins. On both sides of the lower Snake River 

 the bluffs of the Payette formation attain a height of over 800 feet. In the Payette Valley 

 south of Emmett the sharply defined bluff of Payette beds rises 600 feet above the alluvium. 

 Smaller masses, detached by erosion or uplifts, lie in the intermontanq valleys, as far east as 

 the Idaho Basin. 



Along the Boise Mountains the Payette beds rest against the irregularly eroded and sharply 

 sloping surface of the granite, and the top straturii attains a height of 4,100 feet. A total 

 thickness of 1,000 feet is exposed near Boise, and wells bored show several hundred feet of 

 similar strata below the surface. Over the larger part of its extent the formation Ues nearly 

 horizontal or dips only a few degrees. Near the mountains dips of 8° to 10°, generally west- 

 ward, are noted, and the smaller detached masses in the intermontane valleys are still more 

 disturbed, generally dipping westward at angles up to 50°. 



As might be expected from the character of the land mass from which the sediments were 

 obtained, the latter consist chiefly of granitic, hght-colored sands, locally cemented by hot- 

 spring deposits to hard sandstones or clayey semiconsolidated sandstones. Heavy masses of 

 conglomerates and gravels begin to appear at Table Mountain and reach their greatest develop- 

 ment opposite the mouth of Boise River, in the high ridge extending in a westerly direction. 

 Purely clayey deposits are rarer, occurring only in convenient sheltered locations near the 

 shore line or in places where volcanic eruptions took place. The basal part of the formation 

 contains, at Horseshoe Bend, Jerusalem, and other locahties along the Payette, small coal 

 seams. In the clay accompanying these coal seams vegetable remains are of frequent occur- 

 rence. The following forms were identified by Prof. Knowlton: 



Sequoia angustifolia? Lx. 

 Quercus consimilis Newb. 

 Quercus simplex Newb. 

 Acer trilobatum productum? Heer. 

 Salix anguata Al. Br. 



Platanus aspira? Newb. 

 Ficus ungeri Lx. 

 Ulmus speciosa Newb. 

 Betiila angustifolia Newb. 



• From these data Prof. Knowlton draws the conclusion that the age is Upper Miocene, 

 contemporaneous with the flora of the auriferous gravels and the lone formation of California, 

 the Lamar flora of the Yellowstone National Park, and the John Day formation of Oregon. 

 The paleobotanical evidence confirms the conclusion, confidently drawn from the field work, 

 that all these smaller detached masses of lake beds are of practically the same age. 



During the time of the maximum extension of the Payette Lake its surface stood at the 

 present elevation of 4,200 feet. Its deposits, over 1,000 feet thick near the shore, rested against 

 the abrupt slope of the Boise Mountains and filled the old canyon of the Boise to the same 

 depth. The canyon must have formed a fiord, the branches of which reached as far back as 

 the Idaho Basin, and in which vast quantities of gravel and sand accumulated. Isolated 

 occurrences of well-washed gravel on the summit of high ridges in the lower Moore Creek 

 drainage, at elevations of 4,500 feet, confirm the above conclusions. The data are not at 



