The most ancient rocks of Precambrian age in the Rocky Mountains are in the Wyoming province. These rocks are in Wyoming and adjacent parts of Montana and South Dakota, and may include rocks older than 3 b.y. The older rocks of this province probably are granitized metasedimentary rocks that have been deformed severely and show no evidence of a regional structural pattern. Younger metasedimentary rocks, including iron formations, are in northeast-trending synclinoriums, but even these units are older than 2.5 b.y. Most areas studied in the Wyoming province have had an extraordinarily complex structural history and many areas show evidence of more than one episode of deformation; several areas have northwest fold systems and shear zones that disrupt earlier structure.

The Wyoming province may be bordered on all sides by younger rocks or rocks affected by deformation after 2.5 b.y. Miogeosynclinal rocks of possible middle Precambrian age are along the southeastern border of the province. These rocks may be facies of metasedimentary rocks in Colorado that have been metamorphosed in an event of ~1.7 b.y. Evidence for middle Precambrian metamorphic events also has been found in South Dakota, Nebraska, and Montana. These events cannot be related to regional tectonic trends. For example, Colorado structural trends are east-northeast and northeast, whereas in the Dakotas and Nebraska trends are north-northwest.

Precambrian rocks that are younger than middle Precambrian are primarily west and southwest of the Wyoming province. These include Beltian metasedimentary rocks in Idaho and Montana, and probable Beltian and younger sedimentary rocks in Utah and Nevada. In some areas Beltian metasedimentary rocks have been deformed several times; north-northwest to northwest structural patterns appear in many areas. Some Beltian metasedimentary rocks are probably older than ~1.2 b.y.

Although fragmentary evidence based on partial mapping of the 10% outcrop area of Precambrian rocks in the Rocky Mountains does not allow establishment of firm regional tectonic patterns or make satisfactory correlations of ages and tectonics, evidence to date suggests a remarkably complete sequence of Precambrian rocks may be present from rocks older than 3 b.y. to rocks of late Precambrian age.

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