14 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
beds and the position they occupy — that is, whether they are hori- 
zontal, inclined, folded, broken, or in any way displaced. To obtain 
data of this nature in reference to the Snake River Plains, it is neces- 
sary to stud} 7 the geology of a region having an area of 25,000 square 
miles, but owing to the limited time available only a small part of 
this task was undertaken. 
Work was begun in the region about Boise, Idaho, which had pre 
viously been studied in detail by Dr. Waldemar Lindgren," and carried 
eastward across Elmore, Lincoln, and Blaine counties and into Bingham 
County as far as Blackfoot. The route traversed was as follows: 
Snake River from near the mouth of Bruneau River to Shoshone 
Falls, with excursions southward into Owyhee and Cassia counties; 
from Shoshone Falls northward through Shoshone, and up Big Wood 
River, for about 20 miles; northeastward along the west border of the 
Snake River Plains from Carey to Arco, thence eastward across the 
broad plain surrounding Big Butte and its two lesser companions to 
Blackfoot. At Blackfoot the party was disbanded. Included in this 
report also are certain observations made a year before, during a jour- 
ney from Market Lake westward to Big Lost and Little Lost rivers, 
Birch Creek, etc., and back by the way of Mud Lake. 
The general routes followed during the two journeys outlined above 
are indicated on PI. I. The distance traveled was about 550 miles 
without reckoning numerous side excursions. The area more or less 
thoroughly examined includes about l,20i» square miles. As may be 
judged from these facts, the aim in view was a geological reconnais- 
sance, and not a detailed survey. It is hoped, however, that the main 
object of the examination was accomplished, and that the numerous 
observations made will be of value, if for no other reason, because 
they pertain to a region the geology of which was previously almost 
entirely unknown. 
TOPOGRAPHY. 
The Snake River Plains consist of a generally flat area bordered by 
rugged mountains, and extend in a curved course concave to the 
north entirely across the southern portion of Idaho. The length of 
this belt measured along its medial line is about 350 miles and the 
width is generally from 50 to 75 miles. Its area, as estimated by 
Lindgren, is 34,000 square miles. Snake River traverses this rudely 
crescent-shaped plain throughout its length in a great curve concave 
to the north, which has a radius of some 175 miles. In a far-reaching 
view the flat country through which the Snake River finds its way 
presents the broader features of a vast level-floored stream-eroded 
valley. A nearer acquaintance, however, shows that it is a built-up 
plain, formed principally of lava sheets, and does not owe its major 
surface features to erosion. 
a Geologic Atlas U. S., folio 45, Boise, Idaho, 1898. 
