Page 23 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

22 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N M.
impression is gathered that here are the remnants of old river terraces, which sloped on either side toward the center of the plain and were partly dissected by east-west tributaries, giving an ancient land surface very similar to that of the present Rio Grande valley in the region just to the west. The third feature noted is the presence of a dry playa lake bed in the valley east of Upham, and in general the gentle sloping of the valley floor in all directions toward this lake. To the south these gentle slopes have obliterated all former topography except the lava bluffs, which appear to have been faulted up above the general level of the plain. To the north, older topographic features pre- dominate ; the former river terraces are still visible, although the river and tributary channels are now largely aggraded by the superimposed bolson development, and the terraces have been worn down and rounded off until only remnants are left. These features are believed to support the view of Lee, Gordon and others that the valley is a detrital-filled block-faulted area rather than a synclinal basin ; that it was once the course of the Rio Grande, but that later flows of basalt and possibly some distortion dammed it to the north, thus diverting the river into its present channel ; and that the present bolson topography, which is a recent feature, has become well established in the southern part, but it had not much more than begun to the north, when tributaries developing from the new channel of the Rio Grande cut back through the ranges and inaugurated a new cycle of erosion, which in places is preventing the extension of the true bolson-plain topography.
The valley of the Rio Grande between the southern end of the San Mateo Mountains and the southern end of the county, where it is again restricted in width by outcrops of lava and basalt, is considered to be an alluvial plain. Such plains have been called "gradation plains" by Johnson10 and "clinoplains" by Herrick." Two types of plains are represented in this area. According to Gordon :12
Along the side of the valley resting against the flanks of the mountains is the talus plain, or, as Johnson calls it, the "debris apron." This plain is made up by the confluence of the alluvial fans, together with deposition from sheet-flood erosion along the foot of the bordering mountain slopes. The material composing this plain has been derived from the adjoining heights and its surface has a relatively high grade, the result primarily of the ground absorption of the run-off on meeting the lessened slope of the valley plain.
As stated by Herrick, "in some places the talus plain reaches the bottom, but generally it passes abruptly into the plain of the second type or clinoplain proper." The surface of this plain is but slightly inclined and is broken by arroyos with steep walls and nearly parallel courses, the interstream areas being intersected by an intricate network of small, shallow tributaries. In places this plain also reaches to the bottoms, the
10 Johnson, W. D., The High Plains and their utilization : U. S. Geol. Survey Twenty-
first Ann. Rept., pt. 4, pp. 609-741, 1901.
‘ Herrick, C. L., Clinoplains of the Rio Grande : Am. Geologist, vol. 33, pp. 276-381, 1904. '2 12Gordon, C. H., op. cit. (U. S. G. S. Prof. Paper 68), p. 222.