ACADEMY OF SCIENCES] WHEELER SURVEY 51 



SUCCESSIVE PERIODS OF EROSION 



Regarding the third physiographic generalization mentioned above, it should be recalled 

 that, at the time of Gilbert's early work, few if any geologists had discussed the second upheaval 

 and deeper erosion of a region which had previously broadly degraded in consequence of a 

 long-preceding first upheaval. Uplands of even skyline, but dissected by valleys, were then 

 interpreted, if interpreted at all, as uplifted plains of marine denudation in their first period of 

 subaerial erosion. An intermediate period of subaerial planation between two long-separated 

 uplifts was not conceived; residual hills were regarded as unconsumed remnants of a first uplift. 

 Even in Gilbert's descriptions of the plateau and the basin ranges provinces, no explicit state- 

 ments concerning two uplifts separated by a long period of erosion are to be found; yet certain 

 passages in the second report indicate that such a succession of events had been more or less 

 consciously present in his mind. True, the immense denudation that followed the general 

 uplift of the region and "reduced the plateaus to their present condition" (81) was not at the 

 time of these early surveys understood by Gilbert — or by Powell either for that matter — to 

 have been followed by a later uplift which permitted the much smaller erosional work seen in 

 the Colorado Canyon, huge as the canyon is; Dutton appears to have been the first to have 

 recognized the necessity of a twofold elevation. 



Yet Gilbert made the suggestion that the Aubrey Cliff, which "now rises from three to five 

 miles back from the brink of the canon" in its western part — thus leaving the platform of inter- 

 mediate height which Dutton later called the esplanade— "may be supposed to have retired 

 to that position by slow waste during the excavation of the canon" (81); and if that were 

 understood it would seem that the ten or twenty fold greater recession "by slow waste" of the 

 Triassic and other cliffs to the north ought to have been referred to a precanyon period of erosion 

 when the whole region stood lower; but no such conclusion is explicitly announced. Indeed, 

 a statement made regarding the strong lateral retreat of the Triassic cliffs from the narrow cleft 

 known as the Marble Canyon, cut by the Colorado in the resistant Carboniferous limestones 

 east of the Kaibab (68), permits the belief that the above suggested conclusion had not been 

 reached. Certainly a short statement already quoted in connection with notes on historical 

 geology, to the effect that the plateau province has "been elevated, relatively to the adjacent 

 portion of the Great Basin, not less than 4000 feet since the drainage of the great Tertiary lake" 

 (60), and a similar but briefer statement that "the Plateau region . . . has been bodily up- 

 lifted" (187), give no intimation of two movements of upheaval, separated by a long period of 

 immense denudation. 



A clearer implication of the concept that two periods of erosion were separated by an up- 

 heaval is made in an account of a district near Camp Apache in eastern Arizona, on the border- 

 land of the plateau and basin range provinces. The broadly eroded edges of the Carboniferous 

 and Triassic formations, inclining gently northeastward, are unconformably covered by some 

 500 feet of gravel, and this is overspread by 70 feet of basalt. Post-basaltic erosion has exca- 

 vated "a valley several miles broad and 1200 feet deep" (135). 



The evenness of the basalt sheets that spread over its [the gravel's] original surface, indicate that the forma- 

 tion floored a plain, and suggest a relation of altitudes far different from the present. The region is now so 

 elevated that its erosion is very rapid. Streams have sunk their channels to a depth of two thousand feet below 

 the old plain, and carried the eroded material to the modern plain of the Lower Gila, which lies so little above 

 the ocean level, that its slopes are slightly inclined, and its arroyos shallow" (172). 



An uplift after the formation of the gravel-covered and basalt-capped plain and before the 

 erosion of the modern valleys is clearly implied. The quoted passage is further interesting in 

 containing one of the few references to what would now be called the normal baselevel of erosion. 



Another passage in which a revival of erosion is implied concerns the Acoma and Mount 

 Taylor Plateaus, already referred to. 



The Cretaceous field southwest of the Acoma plateau has been reduced nearly or perhaps quite a thousand 

 feet since the eruption of the Acoma lava, but it has been reduced so evenly, that its surface is now as near level 

 as that upon which the Acoma lava was spread. And the same may be said of the field north of the Mt. Taylor 

 plateau, which has been degraded even a greater amount (554). 



