btjssell.] IDAHO FORMATION. 51 
earth movements and deep denudation since the time the sands, etc., 
were laid down in horizontal sheets in the old lake. No such detached 
areas have been discovered in the mountains to the east of the Boise 
quadrangle. 
From what is known of the Payette formation, it seems that it has 
its greatest extent in southwestern Idaho and neighboring portions of 
Oregon, and while probably present beneath the main portion of the 
Snake River Plains — that is, to the east of Malade and Salmon rivers — 
is there deeply buried, but little disturbed from its original horizontal 
position and not laid bare by erosion. 
The upper limit of the beds deposited in Lake Payette is stated by 
Lindgren to be about 4,000 feet above the sea, but as quite extensive 
movements have occurred since they were laid down, the precise 
horizon of the surface of the old lake is difficult to determine. During 
the later portion of the Tertiary the waters of Lake Payette seem to 
have subsided, and possibly a new lake was formed in the same basin, 
principally in southwestern Idaho, in which sedimentation occurred, 
without, so far as is now known, a well-marked, if any, period of ero- 
sion intervening. The beds laid down in this younger lake and by 
streams discharging their sediments into the same basin, are less con- 
solidated than those of the typical Payette formation, and are mostly 
loose sands and soft calcareous clays, with minor quantities of volcanic 
lapilli and dust and well-rounded gravel. 
IDAHO FORMATION. 
The presence of lacustral deposits in Snake River Canyon of younger 
date than the Payette is thus referred to by Lindgren in the Boise 
folio: 
During the eruptions of the Pliocene basalt the extensive Payette Lake had dwin- 
dled to smaller dimensions. Its shore line for some time probably remained sta- 
tionary at an elevation of 2,700 or 2,800 feet, but in this quadrangle [the area 
described in the Boise folio] there are few marks left of its existence, as most of 
these later lake beds have been obliterated by Pleistocene river wash. Along Snake 
River, from above Glenns Ferry down to near Nampa, late Neocene (Pliocene) lake 
deposits are found as white clays and sands interbedded with thin basalt flows, and 
well exposed in the river bluffs. 
I make the above quotation not for the purpose of controverting 
the conclusion stated, but to record the fact that the observations pre- 
sented below pertain to what Lindgren has decided are the deposits of 
a Pliocene lake, to which the name Lake Idaho was given by E. D. 
Cope. My observations of the abundant exposures of the Idaho forma- 
tions on the borders of Snake River Canyon began on the west at the 
southeastern corner of Ada County and were continued eastward to 
Shoshone Falls. A few typical sections in this region will serve to 
show the nature of the deposits and their relation to the associated 
lava sheets. 
