50 SNAKE RIVER PLAINS OF IDAHO. [bull. 199 
TERTIARY AND RECENT LACUSTRAL FORMATIONS. 
PAYETTE FORMATION. 
During the earlier part of the Neocene (Miocene) period, as deter- 
mined by Lindgren — that is, during Middle Tertiary time — a large 
fresh-water lake occupied the Snake River Basin, and the sediments 
deposited in it are now a prominent feature of the country, especially 
to the west of Salmon and Malade rivers (PI. I). These lacustral 
deposits have been named the Payette formation, for the reason that 
they are well exposed along the lower courses of Payette River. a The 
formation is extensive. As stated by Lindgren, it lies mainly to the 
south and west of the Boise Mountains, and occupies the whole of the 
lower part of the ridge between Boise and Payette rivers, and occurs 
throughout an extensive area in Oregon to the northwest of the mouth 
of the Owyhee River. 
The rocks of the Payette formation are mainly sands, clays, and vol- 
canic lapilli, with occasional beds of coarse gravel, especially near the 
bases of the bordering mountains. The strata have in many instances 
been consolidated, so as to form sandstone and soft shale, but to a 
great extent they are still soft, loose sands and slightly compacted 
sandy clays. At several localities in the Boise quadrangle, and again 
on Schaff ner and Indiana creeks, I found the ordinary sedimentary beds 
to be interstratitied with layers of white volcanic dust several feet in 
thickness. The thickness of the Payette formation, as stated by Lind- 
gren, is over 1,000 feet in the vicinity of Boise, but owing to lack of 
exposures and to the scarcity of deep borings, its maximum depth 
remains to be determined. Its extent, as well as its thickness, sug- 
gests that it is widely spread beneath the Snake River Plains, but 
although it was traced by me eastward from Boise to the region 
crossed by King Hill and Clover creeks, it there disappeared beneath 
lava flows and alluvial deposits and its presence beneath the central 
and eastern portions of the plain remains to be ascertained. In the 
mountains to the north of Boise, Lindgren found several detached areas 
of the Payette formation in which the strata were variously inclined, 
sometimes as much as 50°, showing that there have been extensive I 
"On the map forming PI. I no attempt has been made to separate the Payette and the Idaho 
formations. West of Mountain Home and north of Snake River lake beds and lava sheets arc 
interstratified in such a manner that it is impossible to represent them separately on a map of the 
scale used. The great preponderance at the surface of lake beds over lava sheets, in the region 
referred to, has led to my giving the entire region the color indicating the sedimentary formation. 
South of Snake River the area colored to represent lake beds is mostly Avithout lava sheets. 
The extension of the Snake River lava, shown on the map referred to, eastward from Blackfoot, is 
sketched from geological maps accompanying the Twelfth Annual Report of the United States Geo- 
logical and Geographical Survey of the Territories, by F. V. Hayden, Washington, 1883. All of the 
geological data represented to the west of Garnet are from geological maps published by Waldemar 
Lindgren in the Eighteenth Annual Report, United States Geological Survey, Pt. Ill, Washington, 
1898, PI. LXXXVII, and m the Twentieth Annual Report, Pt. Ill, of the same survey, Washington, 
190U, PI. V1I1. 
