The so-called Muddy Sandstone of Cretaceous age in the northern Rockies is the objective of one of the most intense oil "plays" in North America today. The Muddy is divisible into two lithologically, genetically, and stratigraphically different units. The lower Muddy is equivalent in part to the Skull Creek Shale and consists of channel sandstone, channel-mouth bars, and shallow-water shelf sandstone which were deposited in a regressive sea which was connected with the Gulf of Mexico. The upper Muddy is a sandstone in the Mowry Shale and was deposited on the shallow shelf of a sea which transgressed from the Arctic. The upper Muddy is called "Dynneson," and the lower Muddy has been called "Newcastle" or "Birdhead." The giant Bell Creek field in southeastern Montana produc s from the upper Muddy. Recluse field, in northeastern Wyoming, produces from the lower Muddy.

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