HURRICANE FAULT IN SOUTHWESTERN UTAH S7 



sion. The unequal recession of outcrop cliff on the two sides 

 of a fault is a principle that has already been discussed in con- 

 nection with the plateau region. It was recognized by Powell 

 that when a set of outcrop cliffs is cut by a transverse fault the 

 cliffs on the up-thrown side will be subjected to more powerful 

 weathering and erosion than those on the lower side ; conse- 

 quently they will retreat faster and the outcrops on the two 

 sides will in time become discordant. Professor Davis has 

 recently pointed out that the amount of discordance gives a 

 measure of the length of the period of erosion that succeeded 

 the faulting. In the plateau region the apparently level strata, 

 which really dip slightly to the north, form steep south-facing 

 cliffs which are gradually worn northward along their gentle 

 dip. Since the original Hurricane fault took place the Kanab 

 red sandstone cliffs on the eastern side have retreated over 

 fifteen miles farther than their low-lying counterparts on the 

 western side. Inasmuch as this distance represents, not merely 

 the whole amount of retreat, but the excess of retreat on one 

 side over that on the other, the time during which the process 

 went on must have been very long. 



/) General review of the topography at the end of the inter-fatdt 

 cycle. — The fragments of old surface that are preserved under 

 the lava sheets generally belong to lowland topography, and 

 have been examined chiefly in the subsequent valley that seems 

 to have followed the old fault line, first on one side, now on the 

 other, according as the strata were hard or soft. The mature 

 mountainous hills of the old still unchanged plateau preserve 

 the ancient form of the uplands. Between the highlands and 

 the lowlands were long graded slopes which have now been 

 revived into steep, rapidly receding cliffs of naked rock. All 

 this signifies a long period of erosion — so long that the main 

 valleys were reduced nearl}- to baselevel, although the more 

 remote regions to the north were still somewhat mountainous 

 and there was still considerable relief in the intermediate 

 regions. At that time no Grand Canyon can have existed, 

 although the Colorado river must of necessity have been a very 

 important feature in the topograph}-. Its valley must have 



