18 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bull. 217. 
VALLEYS. 
The valleys of southern Idaho and southeastern Oregon are in gen- 
eral broad with nearly flat bottoms and either trenched by stream 
channels or smooth from side to side. The features due to stream 
erosion are found in the vallej^s which have an avenue of escape for 
their waters, such, for example, as discharge to Snake River, but some 
of those which are not completely inclosed are, on account of the 
aridity of the climate, not scored with stream-cut channels. The 
smaller streams from the uplands, on entering a valley, flow less rap- 
idly than in the upper portions of their courses owing to the flattening 
of their channels. More than this, evaporation decreases the flow, 
and furthermore, also, as the streams enter the arid valleys soil mois- 
ture becomes less and there is percolation away from a stream chan- 
nel instead of into it, as is normally the case in humid regions. For 
these various reasons the streams drop a large part and in many 
instances all of the material they bring from the uplands, and thus 
broaden the valley bottoms and tend to give them even surfaces. 
The material brought out of the usually narrow valleys and canyons 
in the uplands and spread over a wide area in the valleys ranges in 
physical character from coarse gravel and even large stones and 
bowlders to line sand and the finest of silt. The coarser material is 
deposited for the most part near the base of the bordering uplands, 
but under exceptional conditions sometimes is carried far down the 
stream channels, while the finer material, owing to the subdividing of 
the streams in the valleys, and the presence of distributaries on allu- 
vial cones, is spread out over a broad area. By this process deep 
accumulations have been formed over the vallej^ floors. It is to this 
method of deposition that the richness of the soils in the valleys of 
Idaho and Oregon, in common with nearty the entire arid region, is 
due. The streams from the uplands which fail to reach through 
drainage lines to the sea not only deposit their visible loads of sand, 
silt, etc., but their invisible loads as well — that is, the material they 
bring from the higher portions of their courses in solution. This 
process is carried on not only by the ephemeral streams, but to a 
considerable extent, as is well known, by the perennial streams, since 
the latter, while flooded, in many instances widely overspread their 
dry-season channels. 
In southwestern Idaho and in the northern portion of Malheur 
County, Oreg. , the broader valleys are to a great extent floored with 
fine, nearly white silt and volcanic dust — the sediment of Lakes Idaho 
and Payette — deposited in Tertiary time, and in these instances it is 
sometimes difficult to distinguish the older deposits from the stream - 
deposited silts laid down on them at a later date. In fact, the mate- 
rial in each instance, to a considerable extent, is essentially the same,' 
since modern streams have eroded the Tertiary terranes and rede- 
posited the loads thus acquired on adjacent surfaces. 
Throughout many of the valleys of southeastern Oregon the under- 
