4 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The Sevier-Toroweap Fault. 
Previous Statements. — The western part of the Plateau province is 
traversed by a series of monoclinal flexures and faults whose general 
course is from north to south as shown on sheet III of Dutton’s Grand 
Canyon Atlas. Parts of two important faults are now to be described : 
the first is presented in this chapter under the double name given above; 
the second is the Hurricane fault, to which the next chapter is devoted. 
In Dutton’s reports, the Sevier fault is traced southward from San Pete 
valley about two hundred and- twenty miles to an ending near Pipe 
spring. Then after an unfaulted interval of twenty-five miles, the 
Toroweap fault is begun at a point about twenty miles north of the canyon, 
and continued closely in line with the prolongation of the Sevier fault 
to a doubtful termination twenty-five miles south of the canyon. These 
faults and their fellows are associated by Dutton with the Pliocene up- 
lift of the region (see references in my previous article, b, p. 114), 
indeed, with so recent an epoch in the history of the plateaus that “‘ every 
fault in the district is accompanied with a corresponding break in the 
topography.” No instance is recalled by Dutton where “ the lifted beds 
are planed off by erosion, so as to make a continuous level with the thrown 
beds” (b, p. 130). It has seemed to me, however, that this generaliza- 
tion is incorrect; that the faults are much older than the general 
(Pliocene) uplift of the region; that in several instances the existing 
escarpments are not the immediate product of faulting, but are the result 
of erosion on a faulted mass which may have been essentially baselevelled 
in the cycle of erosion initiated by the faulting, and which now shows 
escarpments along the fault lines because of differential denudation in 
a later cycle; and that the absence of an escarpment between the sup- 
posed ends of the Sevier and the Toroweap faults is therefore not suffi- 
cient evidence of the independence of these two great displacements. 
It is certainly true, as will appear in the sequel, that relatively recent 
movement has taken place on some of the faults, so that the more resist- 
ant rocks still stand up in bluffs worn but little back from the fault line ; 
an example of this kind will be described where the Toroweap fault 
crosses the canyon, and a much more imposing example is found in the 
northern part of the Hurricane fault. But there are often no signs of 
recent movement, and a large amount of erosion even in the stronger 
rock masses has usually been effected since their displacement occurred. 
The various lines of evidence that indicate a considerable antiquity for 
