IOWA ACADEMY OF SCIENCE 
151 
SIGNIFICANCE OF THRUST-PLANES IN THE GREAT BASIN RANGES. 
BY CHAEELS E. KEYES. 
For more than a generation no other hypothesis of the rearing of the 
mountain ranges of the Great Basin has been countenanced except that of 
simple normal faulting on a gigantic scale. This is the explanation offered 
by Gilbert*. It has its foundation in the observations made by this investigator 
in the Grand Canyon region of Arizona. 
While faulting is widely known in the Great Basin country, it is one of the 
most surprising of facts that few fault-lines have ever been discovered at the 
foot of the Basin ranges. Whenever the greater lines of displacement are 
located it is almost invariably the case that they are not at the base of the 
mountain ridges, but at a distance of several miles out on the adjoining plaint. 
The mountains are in the main symmetrically developed. The longer ridges 
have usually a distinct bilateral symmetry. These facts are suggestive of 
some inattention to the time-factor of the major faulting. In the majority of 
cases in which critical evidence has been adduced the dates of the principal 
displacements are not nearly so recent as had been assumed. Instead of 
being quite late geologically the period appears to be quite early. Probably 
most of the more profound faulting took place in Jurassic times. While with- 
out question fault movements^ have been continuous since the beginning of 
Mesozoic times, and are even now in progress as Spurrt has lately noted, the 
main displacements are ancient. 
Recent investigations have had especially in mind the determination of the 
dates and of the kinds of the various crustal adjustments in the Great Basin 
country. On the whole, the later movements of the great rock-masses appear 
to be largely the result of compressive rather than of normal strains. The 
infolding of old lava-sheets in deposits of Eocene, Miocene, and Pliocene ages, 
in the Death Valley district of eastern California, for instance, is particularly 
suggestive. Similar marked flexing of Tertiary strata is now known in many 
other places in the same region. 
The compressive character of the acquired geologic structures is especially 
well indicated by certain enormous thrust-planes which have been recently 
noted. Although the exact dates of some of these phenomena are not as yet 
clearly flxed the positions which some of them occupy seems to point to the 
fact that the accompanying movements may have had much to do with the 
rearing of the mountains themselves. The importance of the recognition of the 
presence of thrust-planes in many of the desert ranges lies in the inference that 
*Geog. and Geol. Expl. and Surv. W. 100 Merid., Prog. Kept., p. 48, 1874. 
tBull. Geol. Soc. America, Vol. XIX, p. 63, 1908. 
IBull. Geol. Soc. America, Vol. XII, p. 217, 1901. 
