124 SNAKE RIVER PLAINS OF IDAHO. [bull. 19ft 
mouths, like Bennett, King Hill, and other creeks, have been able to 
keep pace with the down-cutting of the Snake, which is still working 
on hard basalt at several localities below the point where these trib- 
utaries enter. 
The result of the concentration of energy in narrow belts at the bot- 
toms of the streams has been to dissect the portion of the Snake River 
Plains situated west of American Falls, and to form a great system of 
canyons, with broad uneroded and nearly Hat areas between. This 
portion of the region furnishes admirable examples of what has been 
termed young topography. Within this region, however, where the 
soft lake beds are not protected by a surface sheet of lava, as over 
great areas in the northern portion of Owyhee County, even weak and 
ephemeral streams have been able to advance rapidly with their work, 
and much more mature topographic forms occur. 
In the broader and most characteristic portion of the Snake River 
Plains, lying north and west of Snake River and east of Little 
Wood River, the surface is uncut by streams, and is still a vast 
uneroded plain. This tract, about 10,000 square miles in area, is an 
admirable illustration of a new land formed by lava outflows. It has 
been left with its original or constructional surface, in part because 
of its actual youthfulness, as measured in years, but it also owes its 
preservation more definitely to the small rainfall, which fails to supply 
the streams coming from the north and west with sufficient water to 
enable them to reach Snake River. This failure is due in part to the 
open structure of the lava itself, which favors subterranean rather 
than surface drainage. In this connection consideration must be 
given to the fact that the Snake in its upper course is supplied with 
more sand, gravel, and other material than it can cany across the 
plains, where its gradient becomes less than in the mountains to the 
east, and deposition and the formation of broad alluvial fans and 
gravel flood plains on the surface of the lava are in progress. 
As the portion of the Snake in southwestern Idaho, where it flows 
over soft sands and clays, is from 2,200 to 2,500 feet above sea level, 
it is evident that a sill of hard rock exists lower down stream, which 
has regulated the depth to which the river has been able to deepen its 
channel in these soft deposits. Below the mouth of the Payette the 
Snake flows through one of the most magnificent canyons in North 
America, one exceeded in grandeur of scenery only by the Grand 
Canyon of the Colorado. Although this region has not been carefully 
studied and is, in fact, almost entirely unknown to geologists, such 
information as is in hand indicates that a mass of resistant rock has 
been elevated athwart the course of the Snake, and that the task of 
sawing a canyon through the obstruction is still in progress. 
Without wishing to invite the reader to take too comprehensive a view 
of the later geographical history of the region under discussion, one 
