Page 24 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 23
bordering bluffs varying in height with the fluctuations in the course of the stream. Along the border of this plain occur parts of a degradational plain formed by the river at some former stage of its existence. A prominent remnant of this plain occurs on the west side of the Rio Grande in the vicinity of Cuchillo, where it has a width of several miles, and in it head many small ravines and arroyos that lead to the river. The inclination of this plain is less than that of the upper plain. Its level varies according to location, but in general it appears to be about 100 to 150 feet below that of the outer plain. Its height above the river bottoms is apparently a little more. Toward the south the alluvial plain laps about the south end of the Mimbres (Black) Range and merges with the great plain which extends westward to the Arizona line.
East of the Black Range and west of the Sierra Cuchillo and the Animas Hills is a series of three small, nearly enclosed basins filled with alluvial material, which in their original con- dition constituted true bolson plains. The original surface of these plains was covered in many places by flows of basalt and was subsequently dissected by the tributaries of the Rio Grande, which cut deeply into the gravel deposits, leaving remnants here and there in the form of lava-capped buttes. The most north- erly of these basins, in reality, is the southern portion of a much more extensive bolson plain in Socorro County.
GEOLOGY
THE ROCKS GENERAL FEATURES
The rocks of Sierra County include sedimentary, igneous and metamorphic varieties. The sedimentary rocks range in age from Cambrian to Recent, and with the exception of the Triassic and Jurassic, all the intervening periods are irepre- sented. The stratigraphy, both generalized and for each mpor- tant mining district, is shown in Plate II. Unconformities at which rocks of certain ages are absent are indicated by breaks in the geologic column. These breaks are all drawn of conventional size, and hence do not accurately represent the time intervals of erosion or non-deposition, which account for a considerable part of total geologic time.
The descriptions of the various rocks found within this region have been abstracted in large part from the more detailed descriptions by Gordon13 and Darton.14
PRE-CAMBRIAN ROCKS
The prevailing pre-Cambrian rock is a coarse red granite in
which the individual grains may be as large as a centimeter in diameter. Crystalline schists and granite gneisses are prominent in these rocks in places and are usually enclosed in the granite.
13 Lindgren, Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mex- ico: U. S. Geol. Survey Prof. Paper 68, 1910.
14 Barton, N. H., "Red Beds" and associated formations in New Mexico : U. S. Geol. Survey Bull. 794, 1928.