Abstract

This paper presents some of the results of a geologic field investigation, carried on during the past several years, of much of the White River uplift in northwestern Colorado. Several geologists of the U. S. Geological Survey assisted in the field work. The Survey's paleontologists identified many fossil collections and made many of the collections in the field. Dr. Stuart A. Northrop was associated with the speaker for parts of three field seasons and is co-author of the report in preparation on the area from which this paper was taken.

The western one-half of Colorado is characterized by four northwestward trending major anticlines, whose crests are ridges of Precambrian rocks forming the principal mountain ranges of the State. The White River uplift with its southeastward extension, the Sawatch Range, largely comprises the third of these major anticlines west of the Great Plains.

The Colorado and Eagle Rivers are in the southeastern part of the White River uplift and the South Fork of White River crosses the northern part. Glenwood Canyon of the Colorado River is a spectacular canyon in the southern part of the area. Glenwood Springs, located at the mouth of the canyon, lies at an altitude of 5,700 feet. A plateau known as the Flat Tops, whose altitude is about 11,000 feet, comprises most of the northwestern two-thirds of the area.

A total thickness of about 20,000 feet of sedimentary rocks ranging from Cambrian to Eocene in age is present. A generalized description of the exposed rocks is shown in .

It is noteworthy that two small volcanic flows near the Colorado River are much younger than the others. The one in Eagle River valley, which is crossed by U. S. Highway 6, is reported by Landon² to be of post-Wisconsin age. All of the higher parts of the uplift were occupied by ice as late as the Wisconsin stage. All deep valleys, except that of the Colorado River, were glaciated. Thick ice descended southward from the uplift in the main tributaries of Colorado River but terminated several miles north of the river.

Structurally the White River uplift is a broad dome that is somewhat elongate northwestward. The rocks on the south and southwest flanks of the uplift dip steeply, 50 degrees to vertical; the dip of the rocks on the northwest flank of the uplift is moderate, only 8 to 10 degrees, and similar moderate dips are present on the east and northeastern flanks of the uplift. Most of the area is a high plateau on which the rocks have only gentle dips. The main folding and faulting of the region was accomplished prior to the outpouring of the basalt.

The uplift is characterized by numerous faults, many of which trend slightly north of west. The faults that have had the greatest structural effect are a few reverse faults whose planes in the southern one-half of the area dip northward and in the northwestern part of the area dip westward. The few data available suggest that the fault planes dip at angles of less than 10 degrees. The stratigraphic displacement of some of these faults ranges from 1,200 to 1,500 feet and that of one fault near the southeastern corner of the area ranges from 3,000 to 4,000 feet. The blocks between the reverse faults contain many normal faults trending northwestward, having displacements of 50 to 400 feet and downdropped on the northeast. These normal faults probably formed at the time of, or shortly after the elevation of the blocks between the reverse faults. The available data suggest that the White River uplift is the result of compression that acted in a general easterly to southeasterly direction during the Laramide revolution.