The long, north-south-trending Front Range in Colorado is a complex mosaic of Precambrian crystalline rocks bordered on the eastern flank by a narrow eastward-dipping foothills belt. The sedimentary formations have a total thickness of nearly 3 miles.

The Precambrian consists of high-rank metasedimentary formations, quartzites, and layered meta-igneous rocks. About one third of the area is composed of folded and faulted metamorphic formations and nearly two-thirds is occupied by at least six generations of competent granitic rocks. Small batholiths, stocks, and innumerable plutons intrude the metamorphics, constituting major, fairly rigid bulwarks of the anticlinal structure of the Range.

The sedimentary formations are of every geologic age except Silurian and possibly Devonian. There are four major lithologic units: (1) older Paleozoic marine limestones and sandstones, (2) Paleozoic and Mesozoic dominantly terrestrial and littoral redbed and arkosic formations, (3) marine Cretaceous formations, and (4) Tertiary formations of continental type.

Five fault-bounded, northwest-trending segments extend across the range and involve the foothills belt of the eastern flank. The segments follow structural trends established in Precambrian time and reactivated during the Laramide. The eastern margin of the Range and foothills belt is underthrust and upthrust for about 90 miles along general north-south trends. Cross folds, tear faults, grabens, horsts, en échelon faulted folds, and drape folds dislocate the foothills formations and the Precambrian rocks adjacent to the foothills belt.

Fifteen structural units within the eastern foothills belt and associated Precambrian formations are described. The distinctive units are the result of strong horizontal compression from east to west during subsidence and underthrusting of the bordering basins in Laramide time, compensating uplift of the body of the Range, and minor deformation of the more plastic foothills formations.

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