104 GROVE KARL GILBERT— DAVIS [MEMOIKS [^ uxx£ 



SUBSEQUENT VALLEYS IN THE HENRY MOUNTAINS 



In association with the above citation concerning the radial streams of Mount Ellsworth, 

 Gilbert wrote: 



It is usually the case, where the strata which incline against the flank of a mountain are eroded, that the 

 softer are excavated the more rapidly, while the harder are left standing in ridges; and an alternation of beds 

 suitable for the formation of a ridge occurs here. One of the upturned beds is the massive Vermilion Cliff sand- 

 stone, and beneath it are the shales of the Shinarump group. By the yielding of the shales the sandstone is 

 left prominent, and it circles the mountain in a monoclinal ridge. But the ridge is of a peculiar character, and 

 really has no title to the name except in the homology of its structure with that of the typical monoclinal ridge. 

 It lacks the continuity which is implied by the term "ridge." 



Then follows the passage already quoted regarding the 15 radial streams, after which it 

 is added: 



Each of these cuts the ridge to its base, and the effect of the whole is to reduce it to a row of sandstone 

 points circling about the mountain. Each point of the sandstone lies against the foot of a mountain spur, as 

 though it had been built for a retaining wall to resist the outthrust of the spur. Borrowing a name from the 

 analogy, I shall call these elementary ridges revet-crags, and speak of the spurs which bear them as being revetted 

 (25,26). 



Evidently, the little subsequent valleys which head against each other in short pairs behind 

 the prominent revet crags were not conspicuous enough to be worth naming. 



The same conditions obtained around Mount Pennell, although the ridge is there more 



continuous: 



i 



The Gate sandstone has been worn away nearly to the foot of the slope, and forms a monoclinal ridge circling 

 about the base. The ridge is interrupted by a number of waterways, and it sends salients well up upon the flank, 

 but it is too continuous to be regarded as a mere line of revetments (36, 37). 



Yet nothing is here said of the monoclinal valleys that the successive arcs of the encircling 

 ridge inclose. Although all such monoclinal valleys were understood to be excavated along 

 belts of inclined shales, it was not explicitly stated that their excavation was accomplished 

 by the headward erosion of wet-weather branches of the radial streams; nor was it noted that 

 each radial stream therefore has a pair of such branches mouthing opposite each other, and 

 that each monoclinal depression back of a revet crag has a pair of such branches heading against 

 each other. 



It thus seems that the failure to give more explicit consideration to the short monoclinal 

 subsequent valleys around the Henry Mountains domes led also to leaving Hoxie Creek and its 

 30-mile monoclinal valley unexplained; both when it was earlier called "consequent" and 

 when it was later bereft of a genetic name. And yet its explanation was unconsciously pre- 

 pared for in two well-considered statements; one regarding the tendency of waterways to 

 follow weak belts, and the other regarding the processes by which this tendency is realized. 

 As to the first it is said: 



In a region of inclined strata there is a tendency on the part of streams which traverse soft beds to continue 

 therein, and there is a tendency to eliminate drainage lines from hard beds. 



However/ — 



The tendency of waterways to escape from hard strata and to abide in soft, and their tendency to follow 

 the strike of soft strata and to cross hard at right angles, are tendencies only and do not always prevail (135, 137). 



The second statement concerns the process by which streams are developed along mono- 

 clinal belts of weak strata, and for which full explanation is given under the discussion of the 

 instability of divides, where it is shown that: 



In homogeneous material, and with equal quantities of water, the rate of erosion of two slopes depends on 

 their declivities. The steeper is degraded the faster. It is evident that when the two slopes are upon opposite 

 sides of a divide the more rapid wearing of the steeper carries the divide toward the side of the gentler. The 

 action ceases and the divide becomes stationary only when the profile of the divide has been rendered sym- 

 metric. ... It results also that if one of the waterways is corraded more rapidly than the other the divide 

 moves steadily toward the latter, and eventually, if the process continues, reaches it. When this occurs, the 

 stream with the higher valley abandons the lower part of its course and joins its water to that of the lower 



