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The Three Forks quadrangle at the head of the Missouri River, has a long stratigraphic succession. The oldest rocks are Precambrian gneiss, derived from arkosic sandstone, and amphibolite, probably altered from mafic sills and lava flows.The unmetamorphosed sedimentary rocks range in age from late Precambrian to Recent, have an aggregate thickness of about 11,000 feet, and underlie about 85 percent of the quadrangle. The only systems not represented are Ordovician, Silurian, and Triassic. The oldest unmetamorphosed sedimy'1tary rocks are at least 4,000 feet of marine micaceous arkose and conglomerate, the North Boulder formation of the Belt series of late Precambrian age. The formation is a wedge arkose controlled by a buried east-west fault of pre-Belt or Belt age here named the Willow Creek fault. l Tonkinella cf. T. stephenensis Kobayashi Kootenia sp. (with 10 long marginal spines) Bolaspis sp. indet.

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