OF-03-15 Geologic map of the Taylor Ranch Quadrangle, Costilla County, Colorado


Map shows distribution of various rock types and their configuration and provides information about potential resources and hazards. Includes cross section, map unit correlation, shaded-relief map with geology overlay, booklet of extended descriptions of map units, economics, geology and selected references. 30 pages. 1 color plate (1:24,000). Digital ZIP download. OF-03-15D

From the Author’s Notes:

San Luis Valley lies in the Southern Rocky Mountain physiographic province as defined by Fenneman (1931). It is one of a series of linear valleys stretching from west Texas, through New Mexico, and into Colorado nearly to the border with Wyoming. The Sangre de Cristo Mountains are on the eastern side of San Luis Valley, and the San Juan Mountains form the western side of the valley in Colorado. A prominent block of steep, rugged, mountainous terrain in the Sangre de Cristos north of the quadrangle, which includes the 14,345-ft high Blanca Peak, is referred to as the Blanca Massif.

Upson (1939, 1971) subdivided the San Luis Valley into five distinct physiographic subdivisions. The northern part of the valley, which Upson referred to as the Alamosa Basin, is north of a roughly southwest-oriented line that runs the Blanca Massif to Antonito. The San Luis Hills, which are relatively low and subdued compared to the Sangre de Cristo Mountains, are south of this line and have almost 1,000 ft of local topographic relief. The San Luis Hills separate most of the Alamosa Basin from the remainder of the valley. To the south, in New Mexico, a broad undulatory surface with local rounded hills and mountains and steep-walled, narrow, incised valleys characterizes the Taos Plateau. The eastern margin of San Luis Valley forms a reentrant from south of the Blanca Massif to a short distance south of the Colorado-New Mexico state line. Upson (1939) named this recessed area the Culebra Reentrant. The portion of the Sangre de Cristo Mountains that is east of the reentrant is called the Culebra Range. The geographic boundary between the Culebra Reentrant and the Culebra Range extends roughly north to south through the quadrangle.

The Taylor Ranch quadrangle extends from the subdued foothills of the western side of the Culebra Range eastward nearly to timberline. Culebra Creek flows from east to west across the central part of the quadrangle. El Poso Creek, a major tributary to Culebra Creek, enters the quadrangle near its northeastern corner and flows southwestward to its confluence with Culebra Creek near the center of the quadrangle. North Vallejos Creek flows westward along the southern edge of the quadrangle. All three creeks have headwaters that extend to the crest of the Culebra Range. Rito Seco (Creek) obliquely crosses the northwest corner of the quadrangle.

The hills that lie between these valleys have gently to moderately steep slopes and attain a maximum elevation of about 11,300 ft in the northeast part of the quadrangle. Outcrops in the hills are relatively sparse and of limited extent. Only the more erosion-resistant igneous rocks form reasonably good, fairly continuous exposures; the sedimentary formations generally only crop out in stream banks and road cuts. The Proterozoic rocks form prominent cliffs in the glaciated upper reaches of El Poso Creek, but elsewhere they also are poorly exposed.

The southwest corner of the Taylor Ranch quadrangle lies within a prominent, regionally extensive, north-south-trending topographically low area. Stevenson (1881) named this valley Culebra Park, a practice followed by Upson (1939, 1971). Culebra Park starts in New Mexico near where Rio Costilla flows out from the Sangre de Cristo Mountains, runs generally northward about 25 mi, and then swings northwestward for about 20 mi, past the town of Blanca to where it merges with the Alamosa Basin. Rio Costilla has incised about 500 ft through uplifted basalt flows on the western margin of Culebra Park, but northward the park floor is generally about at the level of modern streams, including where it cuts across the southwest corner of the quadrangle.

The small villages of Chama and Los Fuertes (also called San Isidro), located in the southwest part of the quadrangle, are the only populated towns in the mapped area. Others who reside in the quadrangle live on ranches or in subdivisions. The mill and tailings impoundment for the San Luis Mine, operated by Battle Mountain Resources, Inc., is in the northwest corner of the mapped area. Production ceased here in the 1900s, and the property now is being reclaimed. Flood-irrigated fields along Culebra, El Poso, and Vallejos creeks provide forage for livestock, and much of the adjoining dry-land hill country has been grazed historically. County-maintained public roads follow the valleys of Culebra Creek, El Poso Creek, Vallejos Creek, and Rito Seco, but public access eventually ends before the roads reach the east side of the quadrangle.Unpaved subdivision roads serve portions of the western and northeastern parts of the quadrangle.