54 E. HUNTINGTON- J. IV. GOLDTHWAIT 



THE INTER-FAULT OR PLATEAU CYCLE OF EROSION. 



After the first Hurricane faulting came a long period of 

 erosion, during which the region as a whole was reduced to a 

 condition of moderate relief and the original topography due 

 to folding and faulting was almost entirely effaced. Near the 

 main drainage lines there was an approach to baseleveling, and 

 the topographic effect of hard and soft strata was largely lost. 

 The initial fault scarp was entirely effaced, and the line of dis- 

 placement came to be marked by an escarpment on that side 

 where hard strata overlooked soft. At the same time the south- 

 facing cliffs on the two sides of the fault retreated northward at 

 different rates. In the more northern district, with the head- 

 water development of a large drainage system, a rounded sub- 

 mountainous topography was produced. Everything indicates 

 that the interval of erosion that followed the first faulting was 

 very long. At the end of the period the lowlands were strewn 

 with waste. Finally came a time of volcanic activity, when 

 craters were formed and the old land surface was partly sheeted 

 over with basaltic lava. It is chiefly to these basalt flows that 

 we owe our knowledge of the two periods of faulting and of 

 the interval between them, for they have preserved the old low- 

 land surface. This is exposed in many places, especially where 

 the lava sheets have been traversed by the later fault. 



a) The lava- covered Hiirrica7ie fault at Coal Spring. — Button 

 states that near the Colorado river the Hurricane fault splits 

 into four branches, two of which must be of recent date, since 

 they are described as cutting recent lava beds. The other two 

 are not known to cut any lava. At Coal Spring, however, twenty- 

 five miles north of the Colorado canyon, where no branches have 

 been observed, an unbroken sheet of lava, showing no sign of a 

 new fault, lies across what seems to be the main Hurricane fault, 

 preserving a surface which is level in spite of the fact that the 

 down-thrown side consists of soft Verkin shales and the other of 

 hard Aubrey limestone. Since at the time of the basalt flow hard 

 and soft strata lay side by side with an almost level surface, this 

 part of the fault must have occurred so long before the flow that 



