GEOLOGIC MAP OF THE SILVERTON AND HOWARDSVILLE QUADRANGLES, SOUTHWESTERN COLORADO

The Silverton and Howardsville 7. 5-minute quadrangles (Luedke and Burbank, 1996a,b) are located in the western San Juan Mountains of southwestern Colorado (fig. 1 and map sheet). Together they include about 305 km (110 mi) of rugged mountainous terrain at altitudes ranging from about 9, 000 ft in the Animas River canyon, at the south edge of the map, to 13 552 ft on Tower Mountain in the north-central part. The Co~tinental Divide crosses the southeastern corner of the map area with the headwaters of the eastward-flowing Rio Grande in the basin just east of Canby Mountain; most of the map area, however, is a part of the westward-draining Colorado River system. Steep valley walls below timberline (about 11,500 ft), particularly many north-facing slopes, usually are covered with dense timber and underbrush; geologic features may be obscured by this cover and, locally, by a veneer of iron-cemented rock debris and overlying soil. Howardsville, the first of the very few towns to be sited within the map area, is located in the Animas River valley at its confluence with Cunningham Creek. For a short period of time in the middle 1870's, it was the county seat of the newly formed La Plata County in southwest Colorado. In its heyday in the 1880's, the town had a population of about 300 people, most of whom were engaged in mining and related industries (Ayers, 1951). A few kilometers to the west, also in the Animas River valley at its junction with Mineral Creek and located on the flats locally known as Bakers Park, is the town of Silverton, which now is the only commercial center within the map area. Silverton became the county seat of San Juan County when formed from La Plata County in 1878 (Henderson, 1926). In the ensuing years, the town survived the fluctuating economics of the mining industry. Although mining activity is currently at a low ebb, the town today is a thriving, active community. During the summer months, it is the midday terminal of the DurangoSilverton narrow-gauge railroad, a popular tourist excursion. The earliest areal geologic mapping in this area, done by Cross and others (1905), subsequently was included in more comprehensive reports of the entire San Juan Mountains region by Cross and Larsen (1935) and Larsen and Cross (1956). Prior to that time, most articles and reports on the geology of the area were of a general nature only, but included a few notable reports (Endlich, 1876; Comstock, 1883; Rickard, 1903). The first