120 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
basalt from 10 to 25 feet high. It trends east and west and can be 
distinctly traced for half a mile or more. A view of a portion of this 
wall is shown on PI. XXIII, A. This fatdt scarp faces south, and the 
adjacent portion of the sheet of obscurely columnar basalt that was 
broken across, and upraised, has a slight dip to the north. There is 
no open fissure at the base of the escarpment, for the reason, probably,, 
that the change in the topography produced by the fault caused the 
line of fracture to become occupied by a small stream which flows 
during the winter or when snow is melting. 
The open fissure at Cleft, which changes to a fault when traced east- 
ward, belongs to a series of recent faults, which are best displayed in 
the portion of the Snake River Plains that lies between 5 and 9 miles 
north of Mountain Home and between Canyon Creek on the west and 
Rattlesnake Creek on the east. The northern margin of this broken 
tract of old lava is formed by the precipitous southern or southwest- 
ern border of rugged mountains of rhyolite. In the area just referred 
to there are five principal faults, each trending about east and west, 
and presenting steep and in part vertical scarps of basalt facing south. 
The heaved block, the broken edge of which forms a bold escarpment, 
looking like a wall when seen from the south, is in each case inclined 
downward to the north and maintains a uniform or slightly decreasing 
dip to the foot of the next escarpment. The height of these walls 
varies from a few feet at their ends to 10 or 50 feet and in several 
localities reaches 150 feet or more. The most northerly of the series 
is close to the base of the mountains, and appears in the wall of the 
canyon cut by Canyon Creek as a broad, irregular cleft. The moun- 
tain face is itself a vast fault scarp, as has already been stated, and it 
is probable that a recent movement along this break in the nature of 
an elevation of the block forming the mountains has caused the com- 
paratively small faults in the lava forming the adjacent portion of the 
plain. 
The displacements to the north of Mountain Home are admirable 
examples of step faults, as they are termed — that is, a series of nearly 
parallel breaks or cracks — separating the rocks they traverse into long 
narrow blocks which are tilted in one direction. The breadth of the 
gently tilted blocks in the instance before us is in general about 1,000 
feet. The faults are not perfectly parallel, but in some instances unite 
or branch, maintaining, however, a general east- west direction, and 
generally coincide in trend with the great escarpment forming the 
border of the adjacent mountains. Some of the faults in the basalt 
extend east of Rattlesnake Creek, but were not followed in that direc- 
tion. On the west the escarpments decrease gradually in height, and 
in most, if not all, instances die out on reaching a dome-like elevation 
in the basalt, described below, through which Canyon Creek has cut a 
narrow, steep- walled canyon. 
