Page 22 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 21
a plain formed by the more or less complete filling of an intermontane basin with detritus derived from adjacent eminences. "Along the margins of these plains are talus hills and fans of boulders and other wash deposits brought down by freshets, while scattered over the interior may be deposits of lacustral origin." The bolson is therefore a constructional plain and is gen- etically a type of gradation plain. As shown by Tight, the essential fea- ture of the bolson is that the plain is bordered by mountain forms or pla- teau escarpments that have supplied the detritus with which it is floored. The definition therefore does not concern itself so much with the forma- tion of the original trough or basin as with the conditions which produce a greater contribution of material than pre-existing drainage can remove. A more or less completely closed basin is, therefore, an essential feature of a bolson plain.
The Jornada del Muerto is a nearly level detrital valley plain 10 to 20 miles in width extending from the latitude of Socorro in Socorro County, through Sierra County to Las Cruces in Dona Ana County, and bounded on the east by the San Andres Range, and on the west by the Caballos and Fra Cristobal ranges. Lee7 states that the altitude of the plain at its north end is about 4,700 feet and at its south end 4,250 feet. There is thus a difference in elevation of 450 feet in 100 miles of length, or an average gradient of 4.5 feet to the mile. According to Gordon:8
The sedimentary rocks exposed in the mountain slopes on either side dip toward the axis of the valley, apparently forming a syncline, and the relations have been described as such in various papers.9 It is doubtful if the structure of the region is as simple as this. In the interior the plain is covered with detritus to a depth of at least several hundred feet, no wells having penetrated its full thickness. Various facts point to the infer- ence that this was formerly the course of the Rio Grande and that, as in other parts of the main valley, the underlying strata have been subjected to block faulting, though no indications of this appear through the heavy cov- ering of detritus.
In the course of the field work which forms the basis for this report, several trips were made through the Jornada be- tween Rincon and Engle, and at a later date the northern end of the plain was inspected. The following observations are believed to be of some significance in connection with the prob- lem. There are several outcrops of lava flows and tuffs in the southern part of the county, which dip in an easterly direction at about the same angle as the older sedimentary beds exposed on the east flank of the Sierra Caballos. However, if a normal sequence of beds lies between these two exposures, then the sediments here are of much greater thickness than has been noted in any other part of the region. It is much more probable that the lava exposures have been elevated to their present posi- tion by block faulting, and have subsequently been eroded to their present low relief. The second point to be noted is that at the northern end of the plain the topography has departed radically from the typical gentle slopes of a bolson plain. The
7Lee, W. T., Water resources of the Rio Grande valley in New Mexico and their devel- opment: U. S. Geol. Survey Water-Supply Paper 188, p. 10, 1907.
8 Op. cit. (U. S. G. S. Prof. Paper 68), pp. 221, 222.
9Keyes, C. Ft., Geology and underground water conditions of the Jornada del Muer'o, N. Mex.: U. S. Geol. Survey Water-Supply Paper 123, p. 26, 1905.