Overall Summary and Selected References

Wayne I. Anderson

The geologic story of Custer County and vicinity goes back nearly two billion years. The oldest rocks of the area are Precambrian in age, and they originated below the Earth's surface under high temperatures and extreme pressures. Many of these igneous and metamorphic rocks formed in ancient mountain belts during the early chapters of Earth History, when bacteria and algae were the dominant forms of life on our planet. Igneous processes during the Cambrian Period also left a distinctive record in the local area.

A partial record of Lower and Middle Paleozoic rocks is found on the west flank of the Sangres and on the eastern edge of the Wet Mountains. These rocks are sedimentary in origin and tell of ancient seas that once covered Colorado.

The Late Paleozoic record is conspicuous in Custer County. Rocks of the Pennsylvanian and Permian systems underlie many of the mountain peaks of the Sangres. Two formations (Minturn and Sangre de Cristo) record the important Late Paleozoic history of the area. The Minturn Formation was laid down in coastal settings and on a marine seafloor (Colorado trough), adjacent to the Ancestral Rocky Mountains. Erosional debris from the Ancestral Rockies was transported by streams and accumulated as alluvial fans at the base of the mountains. Many of the alluvial fan systems built seaward as deltas into the Colorado trough. Sediment-charged currents carried sand and mud seaward and rapid deposition produced graded beds (turbidites). The turbidites generally formed below wave base. During quieter times, limestones with marine fossils formed in shallower settings, above wave base.

Whereas the Minturn Formation records coastal, deltaic, and marine deposition, the overlying Sangre de Cristo Formation is entirely nonmarine. The Sangre de Cristo Formation was stream laid and preserves ancient alluvial fans. The Crestone Conglomerate Member of the formation displays large clasts, typical of settings very close to rugged source lands. Oxidized iron in the sedimentary rocks of the Sangre de Cristo Formation gives a reddish tint to the Sangre de Cristo Range and offers an explanation for the origin of their name. "Sangre de Cristo" is Spanish for "blood of Christ." Some folding and thrusting occured during Late Paleozoic deposition and left its mark in the rock record, too.

The record for early Mesozoic (Triassic) is absent locally because the Ancestral Rockies were still dominant features. Apparently, the Custer County area was subjected to erosion during Triassic time and no record accumulated. Later during the Mesozoic, several Jurassic and Cretaceous formations were deposited. This record is best seen near Wetmore in eastern Custer County and to the north in Fremont County.

The Laramide Orogeny (Late Cretaceous-Early Tertiary) left its signature in the form of complex thrust faults and folded strata. Later, in Middle Cenozoic time, additional faulting and uplift occurred. The Sangre de Cristo Range rose as a block (horst) and the adjacent San Luis and Wet Mountain valleys dropped as grabens. Volcanic activity and mineralization were widespread during Cenozoic time and left significant deposits near Silver Cliff and Rosita.

The Late Cenozoic history of the area was marked by multiple episodes of glaciation and by extensive stream erosion. Running water from melting ice and snow and from precipitation cut extensive erosion surfaces and deposited thick layers of Quaternary alluvium. Faulting has displaced Holocene (Recent) sediments in alluvial fans on the west side of the Sangres. Radiocarbon dates indicate that some faulting took place about 7,500-7,800 years ago. The faults on the east side of the Sangres show no evidence of Holocene displacements, however.

This summary and associated articles were written to provide the reader with some basic geological background to better understand the beautiful mountain landscape of Custer County. We can learn much from the mountains, and it isn't all geological!

The great naturalist John Muir gave sound advice: "Climb the mountains and get their good tidings. Nature's peace will flow into you as sunshine flows into trees. The winds will blow their freshness into you and the storms their energy...while your cares will drop off like autumn leaves." And also from John Muir: "Thousands of tired, nerve-shaken, overcivilized people are beginning to find out that going to the mountains is going home."

Selected References

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Armbrustmacher, T. J., and I. K. Brownfield. 1978. Carbonatites in the Wet Mountains Area, Custer and Fremont Counties, Colorado--Chemical and Mineralogical Data. U. S. Geological Survey Open-File Report 78-177.

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