48 GROVE KARL GILBERT— DAVIS tMB1,0IB8 I ^ , Sst 



Although no general explanation is offered for the origin of these monoclinal valleys, it may be 

 inferred from the context that their excavation by the normal action of degradational processes 

 along the strike of weak strata between resistant underlying and overlying strata was so well 

 understood by Gilbert that it was taken as a matter of course; nevertheless, the failure to make 

 explicit statement of such origin and of the manner in which valleys and streams thus formed 

 replace a preexistent drainage is regretable, for the recognition of an important class of valleys 

 was thereby unduly delayed. The omission may be regarded as a result of the unfortunate but 

 perhaps inevitable inattention to the work of foreign geologists; for Jukes had, 10 years before 

 Gilbert's western work, very clearly explained certain monochnal valleys in southern Ireland as 

 the result of the hcadward or retrogressive erosion in weak strata, "subsequent" to the erosion 

 of the transverse valleys which they join. On the other hand, as Gilbert did not enter into the 

 discussion of valley origin, he was not led into the error made by both Powell and Dutton of 

 regarding manifestly subsequent valleys as the work of antecedent streams. And yet in the able 

 account that Gilbert wrote of "The Colorado plateau province as a field for geological study," 

 above cited, when he takes up the "Problem of inconsequent drainage," he mentions only 

 antecedent and superimposed streams in addition to consequent streams, and gives no sugges- 

 tion whatever that a third kind of "inconsequent" stream may exist. 



PLANATION BY SUBAERIAL EROSION 



The physiographic interpretation of land forms is greatly aided by the recognition of three 

 fundamental generalizations; first, that subaerial erosion will, if continued without interruption 

 for a very long period of time, wear down any land mass, whatever its original structure, form, 

 and height, to a surface of small relief; second, that the forms developed during the progress 

 of uninterrupted erosion will exhibit a somewhat systematic sequence of changes; third, that 

 the continuity of erosional work may be interrupted at any stage of its progress by an upheaval 

 of the land mass concerned, whereupon a new period of deeper erosion will be begun. Although 

 these generalizations had not been formulated at the time of Gilbert's service on the Wheeler 

 survey, they have become well established since then, and all three of them are essential in 

 reaching what is now regarded by those acquainted with the region that he studied as its true 

 interpretation. It is therefore of general as well as of personal interest to inquire how far the 

 above generalizations were either implicitly included or explicitly announced in Gilbert's early 

 reports. 



In undertaking this inquiry it is desirable to remember that, while the importance of subaerial 

 erosion in the production of uneven land surfaces had come to be understood in a general way 

 by the geologists of half a century ago, few if any of them realized that the continued action of 

 erosion would in time extinguish the inequalities of form that it had previously produced. 

 They knew that the erosion of valleys would leave intervalley hills, but they did not perceive 

 that after the valley deepening had almost ceased the erosive processes would be chiefly expended 

 upon the hills and directed to their very slow obliteration. Among British geologists about 

 the middle of the nineteenth century, Lyell gave little attention to land sculpture, although 

 his leading principle of uniformitarianism might have led him far beyond the erosion of mere 

 valleys. Greenwood, a valiant advocate of the efficacy of "rain and rivers" (1857) in the 

 erosion of valleys, marshaled his arguments chiefly against the earlier view, which then still 

 found much acceptance, that valleys had been carved by marine currents; he failed completely 

 to see that valley-excavating processes would in time consume the adjoining hills. Geikie (1868) 

 recognized the possibility that plains of denudation might be produced not only by marine 

 abrasion, as was then generally believed, but also by subaerial erosion; but he usually limited 

 subaerial agencies to the excavation of open valleys, and still regarded waves and currents as 

 the most effective agencies of planation. 



Among American geologists, Lesley, the leader during his earlier years of work on the 

 Alleghenies of Pennsylvania, in explaining the relation of underground structure and surface 

 form (1856), was a catastrophist who ascribed the carving of his open valleys and narrow water 

 gaps to a vast ocean flood, which, rushing southward from the Arctic, worked "with infinite 



