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The Cretaceous of the Rocky Mountain region contains sandstones that were deposited in marine, transitional, and nonmarine environments. Spatial dimensions of sandstones deposited in shallow neritic and transitional environments are regular in character and are easily defined. Only this type of sandstone is here considered, and examples illustrating minimum and maximum geographic distribution are treated.

Minimum size sand bodies are well shown by the Fox Hills sandstone where it is exposed on the northeast flank of the Rock Springs uplift, Wyoming. This formation consists of a series of barrier bar sandstones that change northwestward to lagoonal shales (Lance formation), and southeastward to marine shale (Lewis shale). Detailed surface analysis of one barrier bar shows a thickness of 30 feet and a width of 6–7 miles from the lagoonal shale and sandstone facies to the marine shale and siltstone facies. Each bar is believed to have extended along much of the western margin of the Cretaceous seaway.

The upper part of the Judith River formation of central and eastern Montana exemplifies a transitional and marine sandstone unit having a maximum width. The unit is 140 miles wide and was deposited between lagoonal shale facies to the west and marine shale facies (Pierre shale) to the east. Thickness of the unit ranges from a wedge edge to 100 feet.

The geometric pattern of most of the sand bodies that accumulated along the Cretaceous shoreline is similar in character to the above examples and ranges in size between these extremes.

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