Precambrian metamorphic and igneous rocks, Paleozoic sedimentary rocks, and Tertiary igneous rocks are exposed in the northern Sangre de Cristo Mountains. The sedimentary rocks of Paleozoic age are most abundant and range from Ordovician through Permian. The pre-Pennsylvanian sedimentary rocks average about 1000 feet thick and are for the most part limestones and dolomites with lesser amounts of shale, sandstone, and quartzite. The Pennsylvanian and Permian rocks, more than 14,500 feet thick, are coarse clastic rocks deposited in the Colorado-New Mexico zeugogeo-syncline. The Pennsylvanian rocks are poorly stratified and poorly sorted, drab sandstones and fine conglomerates; red sandstones and arkosic conglomerates predominate in the Permian rocks.

A tonalite stock of Tertiary age intrudes Pennsylvanian rocks in the southern part of the mapped area. Small dikes and sills, probably of Tertiary age, are present locally elsewhere in the area.

The northern Sangre de Cristo Range is a fault block uplifted along a concealed, high-angle fault that separates the range from the San Luis Valley to the west. The sedimentary rocks in the northern Sangre de Cristo Mountains strike generally north-northwest, nearly parallel to the trend of the range, and dip east 30° or more, forming the east flank of the Laramide Sawatch arch. Numerous high-angle strike faults are present. In general, faulting is more prominent than folding. All structures other than those of the Precambrian are probably Laramide or younger in age.

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