40 GROVE KARL GILBERT— DAVIS I ^ MWB " v^xxl 



most wonderful defile" it had been his fortune to behold, he noted that "many times our upward 

 view was completely cut off by the interlocking of the walls, which, remaining nearly parallel 

 to each other, warped in and out as they ascended. " His section of this defile, reproduced on 

 the cover of Leconte's Geology, has been mistaken by some as representing the Colorado 

 Canyon. 



NORMAL HANGING VALLEYS 



Hanging lateral valleys were detected in the narrow incision of Zion Canyon, for many of the 

 side canyons " at their mouths are not cut so deep " as the main cleft, and "discharge at various 

 heights above the river" ; and these hanging side canyons were acutely adduced in an important 

 argument: for beneath their mouths, the sandstones of the main canyon were seen in "perfect 

 continuity and integrity ... a continuity that cannot be seen in the main canon, since its bed 

 is everywhere covered by detritus." The unbroken continuity of the sandstones beneath the 

 side canyons was evidently taken to certify to their similarly unbroken continuity below the 

 main canyon bed, and thus to warrant the interpretation of the canyon not as an example of a 

 fracture, but as "an example — and a peculiarly differentiated exam pi c — of downward erosion 

 by sand-bearing water. The principle on which the cutting depends is almost identical with that 

 of the marble saw, but the sand grains, instead of being imbedded in rigid iron, are carried by a 

 flexible stream of water." Inasmuch as the plateaus were found to be traversed by several great 

 faults, this critical argument for the nonfaulted structure of the heavy sandstones cleft by the 

 Virgin River is all the more pertinent. It is noted further that, as the side canyons are " worn by 

 smaller streams . . . their bottoms are of steeper grade" (79), but it is not explicitly stated, that 

 for the same reason, their depth is less than that of the main cleft; yet this principle was surely 

 understood. In any case, hanging lateral valleys of normal erosion had rarely been described in 

 those days, and had been still more rarely mentioned so understandingly in published reports. 



CATARACTS AND RAPIDS 



Erosional processes and principles were so grandly exemplified by the Colorado River in its 

 canyon that, although many of them were fairly understood by the few explorers of the region, 

 they still merited explicit statement for the benefit of geologists and geographers in general. 

 The extinction of cataracts, for example, is introduced by a reference to the prediction concerning 

 the future of Niagara made by Hall, whom Gilbert had known in Albany, to the effect that 

 "there will come a time when the fall can no longer be maintained . . . and will be replaced 

 by a rapid." Then applying this principle, Gilbert remarks: 



In the Grand and Marble canons [of the Colorado] this stage has been reached, and the whole descent of 

 1,600 feet [along the river] accomplished entirely by rapids. The stratigraphic conditions to the formation of a 

 cataract are indeed not wanting . . . But the river passes the hard beds and the soft with almost equal pace . . . 

 At no place does the river fall from a ledge of rock into a pool below (75). 



Evidently the exceptional cascades found by Powell where dams had been formed by lava 

 streams that plunged down the side walls after the canyon had been eroded can not have come 

 within Gilbert's field of observation. 



The numerous rapids which beset the river, and which, by a literal interpretation of a pas- 

 sage quoted above, might be regarded as almost extinguished, hard-stratum cataracts, are shown 

 to be of other origin. " Some of the most violent rapids" are due to large rock masses which have 

 fallen from the walls, so that they "locally obstruct the channel"; but the majority of the rapids 

 are found, as Powell had noted, where the occasional floods of steep tributary canyons sweep 

 down blocks of rock, sometimes 10 or 15 feet in diameter, and drop them at their mouths, thus 

 half closing the river channel with bowlder deltas or — 



dams, that must often be of great depth. Over each of these the water [of the main river] finds passage at the 

 edge opposite the tributary, and descends the lower slope with swift current and broken surface. . . . To roll, 

 jostle, break, and finally grind up and remove these boulders is the task — perhaps the chief task — of the river, 

 and until it removes them it can perform no work on the solid rock which underlies. ... In the current cycle of 

 events within the gorge [canyonl, there are times when each of these dams in turn is removed. . . . While the 

 dams will occur at the same localities and with the same characters, they cannot be regarded as strictly perma- 

 nent (71). 



