12(5 SNAKE EIVER PLAINS OF IDAHO. [bull. 199. 
The principal variations in the topography of the canyon walls, from 
Shoshone Falls westward, depend on the relation of the lava sheets to 
associated clays, sand, etc., interst ratified with them. Where the lava 
predominates the canyon is narrow with precipitous walls, as in the 
portion of its course for about 15 miles below Shoshone Falls. When 
only a thin surface sheet of lava is present the canyon has flaring sides, 
margined above by a vertical rim rock. If two or more sheets of lava 
are present, with soft beds of considerable thickness between the adja- 
cent sheets, two or more rim rocks similar to the higher one will 
occur, their strength depending on the thickness of the lava. If a 
sheet of lava separated from the highest sheet of the same nature by 
a thick deposit of soft beds is present, a broad outer canyon, with 
flaring sides, and a summit rim rock will usually be developed, in the 
bottom of which there will be a smaller, inner can}^on. When, as 
most frequently happens, the lava sheets occur only on one side of the 
canyon, i. e., on the northern side, the characteristics just referred to, 
with numerous variations, are there present, while the other side is 
less definitely canyon-like and usually presents less well-defined topo- 
graphic forms. 
The slopes formed of soft material below the broken edge of a thin 
lava sheet are usually strewn with blocks of lava that have fallen from 
above, and may be complete^ sheathed with such talus (PI. VII, A)l 
The profiles of these slopes are sometimes concave, as when the talus 
is too thin to preserve the soft beds beneath and erosion curves are 
produced, but may be nearly straight when the slope of the talus is 
the predominant factor, or become convex when the material beneath 
is resistant and weather curves predominate. 
The broken edges of thick sheets of basalt resting on soft deposits, 
especially if the latter are also thick, are apt to break away in larger 
blocks than in the formation of a talus, and landslides result. Excel- 
lent examples of landslides of the nature just referred to occur for 
several miles on each side of Snake River above the mouth of Canyon 
Creek. 
The principal features of the walls of Snake River Canyon, described 
in the last three paragraphs, it must be remembered, are such as are 
most pronounced and become accentuated owing to the prominent part 
played by certain special conditions, but many other variations occur 
which are not essentially different from those known elsewhere in the 
arid region, except in detail. There is one class of exceptional topo- 
graphic forms, however, which becomes especially conspicuous in the 
wall of the Snake River Canyon, the like of which, so far as I am 
aware, has not been observed in other regions. I refer to certain 
remarkable alcoves or branches from the main canyon which are not 
due to the work of surface streams. 
