UPPER ELEPHANT CANYON, containing trail to Druid Arch. (Fig. 53)
DRUID ARCH, from end of arduous trail shown in figures [52] and [53]. (Fig. 54)
After returning to our vehicle west of Chesler Park and backtracking over SOB Hill to the intersection in the middle of Devils Lane, let us proceed northward on a one-way road to and beyond the Silver Stairs for a closer look at Devils Lane and other grabens to the west and for a look at the confluence of the Green and Colorado Rivers. But first let us pause and reflect upon the possible origin of The Grabens.
Geologists have different opinions as to just how grabens and complex systems of joints have formed, but all seem to agree that tensional forces were involved. Some think that solution of salt and gypsum from the Paradox Member of the Hermosa Formation by ground-water movement allowed the brittle Cedar Mesa Sandstone and other overlying rocks to sag, producing tension cracks and faults. Others believe that removal of the salt and gypsum occurred by plastic flowage toward the Meander anticline (see [p. 108] and [fig. 61]), whose axis follows the Colorado River southwest from The Loop, past the confluence, and to and beyond Spanish Bottom. Some suppose that compaction due to the weight of the abnormally thick pile of sedimentary rock underlying the area may have caused the sagging, cracking, and faulting. The rock deformation may have resulted from a combination of these and possibly other things, of course, but whatever the cause, the resulting features are very striking. There was room to show only two of the named grabens within the park on the map ([fig. 1]), but all are shown in [figure 51], and several appear in [figure 59]. A diagramatic cross section of a typical graben is shown in [figure 55]. The tension faults shown in figures [55] and [56] are called normal faults, in contrast to faults formed by horizontal compression, which are called reverse faults (figs. [75], [76]).
The Grabens range in width from about 7 or 8 feet at the north end of Devils Pocket to nearly 2,000 feet at the south end of Red Lake Canyon, but the average width is about 500 feet. The floors of The Grabens are covered by soil and grass, but the displacement along the faults is believed to approximate the height of the walls—nearly 300 feet. That The Grabens are of fairly recent origin is attested by the fact that most of the walls are vertical fault faces showing little sign of erosion ([fig. 57]); that no through drainage has yet been established in Cyclone Canyon, which is a series of basins with low divides between; and that several pre-existing streams were interrupted or diverted by the faulting.
A SIMPLE GRABEN, formed by tension in directions indicated by horizontal arrows. Downdropped central block is the graben; stationary or uplifted blocks on sides are called horsts. From Hansen (1969, p. 123). See also [figure 76]. (Fig. 55)
CUTAWAY VIEW OF NORMAL FAULT, resulting from tension in and lengthening of the earth’s crust. Note amount of displacement and repetition of strata. Compare with [figure 76]. From Hansen (1969, p. 116). (Fig. 56)