academy of sc IENCES] WHEELER SURVEY 47 



It is singular that no mention was made of the analogy between the channel bed on which 

 denuding streams of water flow and the general surface of the land on which degrading currents 

 of air blow; and also that not even Gilbert's philosophic mind seems then to have recognized 

 that, given time, the "ridges and islands " left in the early stages of water erosion must disappear 

 in the later stages, and leave the degraded surface as free from vestigial mountains as if it had 

 been swept smooth by the broadly diligent wind. 



An example of that well-marked group of physiographic forms known as intermont basins 

 was observed between certain members of the basin ranges near the head of the Gila River in 

 southwestern New Mexico. "The building of the Tulerosa and Mimbres ranges separated the 

 basin of the upper Gila from the lower valleys of the stream, and the way which the waters 

 opened for their escape is a profound canon" through the first-named mountains. 



Before the cutting of the canon to its present depth that part of the basin which lies nearest the outlet was 

 filled by erupted and transported materials to a great depth and a lake-like plain was produced, the proportions 

 of which can still be grasped by a bird's-eye view. It was about fifteen miles by twenty-five in size, and sloped 

 very gently toward the outlet. By the deepening of the draining canon, . . . the water-ways have been carried 

 [carved?] below the floor of this plain, and a system of narrow gorges, 500 to 1,000 feet in depth, now traverse and 

 exhibit the filling of the valley. The filling consists of basalt, tuff, and gravel, with nearly horizontal bed- 

 ding. ... In some localities basalt has run into the intersecting gorges since the beginning of their excavation 

 (530). 



The fact that this basin of deformation was called a " valley" and that its branching gorges 

 were characterized as "intersecting" shows that less progress was made in the terminology of 

 land forms than in their explanation. 



Attention to small matters is indicated by the account of numerous low mounds at the 

 eastern base of the Sierra Blanca, near the southern margin of the plateaus, at an altitude of 

 7,000 feet; the mounds are "usually one or two rods broad and less than a foot high, and separated 

 by interspaces several times as broad as themselves. . . . -The grass on the mounds is dis- 

 tinguished ... by a deeper green. Viewed from a commanding position, the effect is singu- 

 larly beautiful, the green spots dappling the plain like the figure of a carpet. . . . There is little 

 question that they [the mounds] are vestiges of hummocks thrown up by prairie dogs, or other 

 burrowing animals ' ' ; but as prairie dogs are not now found on the high plain, " if the mounds are 

 the work of that species, they may point to a climate, in very recent time, of even greater 

 warmth and aridity than the present" (540). 



SUBSEQUENT VALLEYS 



How far Gilbert consciously understood the origin of monoclinal or subsequent valleys is not 

 clear, although he makes repeated mention of them. For example, House Rock Valley "lies 

 in the monoclinal" between the horizontal Carboniferous limestones of the upheaved Kaibab 

 Plateau and the horizontal Triassic sandstones of the Vermilion Cliffs next to the east, the inter- 

 vening monoclinal flexture being occupied by weak beds (53) ; again, a valley is later described 

 "which, along the southern base of the Zuiii uplift, marks the place of the soft Triassic clays 

 between the Carboniferous limestone of the main mountain and the Triassic sandstone" (533); 

 and once more, a "broad flat valley" is said to be the topographic indication of the same clays 

 near Nutria, on the west side of the Zuni uplift (553) . The southwestern border of the plateau 

 province in Arizona, where the strata. of its broad, lowermost "terrace" are gently inclined to 

 the northeast, is drained by several monoclinal valleys excavated along the weak strata between 

 the Carboniferous limestones and the basal Tonto sandstone (60, 80) . A striking example was 

 earlier described in the brief account of the Timpahute Range, an east-dipping faulted monocline 

 in the Great Basin, in which " the quartzites at the west, and the limestones at the east, by their 

 superior hardness, maintain parallel ridges, while the intervening shales have been denuded so as 

 to form a valley within the range," opening southward (38); and similarly in the Santa Rita 

 Range "the weathering of the shale has opened a valley between the outcrops of the limestones" 

 which dip gently to the southwest (516). 



