LINDOBEN.] POST-PAYETTE PHENOMENA. 635 



dips to the west at angles ranging from 10 to 35, the monoclinal 

 uplifts vividly recalling those in eastern Washington described by 

 Professor Russell. 1 There, however, the Miocene beds lie only on 

 top of the volcanic flows of the Columbia formation, while here they 

 lie both below and above similar igneous masses. 



On both sides of the Snake River, from Caldwell to Weiser (near 

 the upper edge of the map, PL LXXXVII), the beds do not contain 

 much evidence of volcanic activity, though from Parma a few vol- 

 canic buttes are visible from the railroad, far to the west, among the 

 lake beds in Oregon. But a few miles beyond Weiser the white bluffs 

 of lake beds begin to assume red or orange colors, and contain streaks 

 of intercalated tuffs. The valley contracts, and at the entrance of 

 the canyon basaltic rocks appear, partly interstratified with the lake 

 beds, partly underlying them. Near Huntington the deep and nar- 

 row canyon is composed entirely of basic volcanic rocks, clearly con- 

 nected with the early Neocene lake beds, and assuredly different from 

 the later basalts from the upper valley. How far down this volcanic 

 canyon extends is not known. 



POST-PAYETTE EROSION. 



After attaining its highest stage, the lake was drained by the 

 establishing of the present course of the Snake River below Weiser. 

 The lake receded as the canyon was rapidly cut by the mighty vol- 

 ume of water, and erosion has steadily proceeded since the end of the 

 Miocene or the beginning of the Pliocene. The broad valleys of the 

 Boise, the Payette, and the lower Snake were eroded in the soft lake 

 beds. A new course was established for the Payette River, which 

 evidently did not debouch at its present position before the Payette 

 epoch. The Boise River, on the contrary, maintained its old course. 

 The accumulated gravels were scoured out from its canyon, and, 

 before the Pliocene basaltic eruptions, its channel in the canyon 

 was deepened nearly to its present level. There was, however, at 

 least one temporary check in this process of draining. For a consid- 

 erable interval of time the lake remained stationary, at a present 

 elevation of from 2,800 to 3,000 feet. The deposits and basalt flows 

 of this epoch are regarded as late Neocene (Pliocene) and belong to 

 the Idaho formation of Cope. 



POST-PAYETTE OROGRAPHIC DISTURBANCES. 



Before the epoch of the Pliocene basalt flows the sediments and 

 flows of the Payette formation were subject to some disturbances, 

 reaching their maximum in the smaller areas in the intermontane 

 valleys. Certain parts of the series acquired a slight westerly dip. 

 More intense orographic movements took place at Squaw Butte and in 

 the Horseshoe Bend and Jerusalem valleys, resulting in monoclinal 



i Bull. U. S. Geol. Survey No. 108, 1893, p. 28. 



