Page 215 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 211
ite base is exposed above the alluvial fans of the region for a vertical height of 350 to 500 feet. The total height of the escarp- ment is between 1,500 and 2,000 feet. The highest point of the range is Timber Peak, which attains an altitude of 10,000 feet. South of Apache Gulch, a small block of Magdalena limestone, dipping 40° N., forms the high ridge along the south side of the canyon. Between the western face of the range and the Rio Grande are alluvial aprons which merge into the valley plains with decided slope to the west and finally terminate in the bluffs of the river channel. The present river valley has been cut in the bottom of an older and wider channel, remnants of which appear as benches along the bluffs and for short distances up the main tributaries. As viewed from a distance, the valley plains give the appearance of having a smooth profile with continuous slope between the foot of the escarpment and the top of the river bluffs. In detail, however, this apparently smooth surface is deeply trenched by steep-sided ravines and arroyos. Near Shan- don the original plain surface has been worn down until only a few rounded hills showing above the benches of the first cycle of erosion remain to testify to its former elevation at this point.
The valley plain is composed of Palomas gravel, overlain by a thin covering of more recent alluvial material. The Palomas gravel rests upon a floor of pre-Cambrian granite and schist near the foot of the escarpment.
Flows of Tertiary lavas, consisting of the usual andesite- latite-rhyolite series, have apparently covered the older Pennsyl- vanian and Permian sediments nearly to the present location of the river. These flows appear to have come from the southeast through Apache Canyon. Residual masses of these rocks pro- ject through or underlie the gravels along the base of the escarpment, and a wide sheet farther south has been faulted down against the main mass of the low ridges.
THE PLACERS
The gold-bearing gravels of the district are chiefly in Tru-
jillo Gulch and the area drained by its tributaries. Two small gold-bearing areas are known also in Apache Gulch. The gold found in Union Gulch, a tributary of Apache Gulch, appears to have been derived from the headward erosion of this gulch into the drainage area of Trujillo Gulch. Farther east, in Apache Gulch, is a small area of workable gravel that was derived from a higher source to the south and east of the main area. (See figure 19.)
Gold was probably discovered at Shandon in 1901. The deposits first became known to the public in 1903, when Encarn- acion Silva, on one of his trips to Hillsboro to dispose of gold from this area, was induced to tell the location of his workings. A stampede to the district immediately followed. During 1904 'Sierra County produced 1,111 fine ounces of placer gold and in