The Groat Sandstone Bed of the Gammon Member of the Pierre Shale (Campanian), equivalent to the Shannon Sandstone Member of the Steele Shale of Wyoming, crops out on the north flank of the Black Hills, where it has been mapped. Subsurface studies in Carter County, Montana, reveal a 3,900 sq km area in which sandstone thickness exceeds 23 m. This area of sandstone includes two small elongate lenses in T6 and 7S, R54 and 55E. The lenses trend northwest and are approximately 16 km long and 6 km wide. In adjacent outcrops, sandstone grades downward and laterally to the northeast into siltstone and then to shale. Sandstone units are less than 15 m thick; near the base they are fine-grained and mottled with clay grading upward into bioturbated medium to fine-grained sandstone, hich then grades upward into medium to coarse-grained sandstone with large-scale, trough cross-bedding.

The Groat Sandstone was probably deposited 322 km from the strandline near an outer shelf margin. This interpretation is based on published strandline positions

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and on regional isopach maps of subsurface units within the Pierre Shale. Local thickening of shale marks the position of the shelf margin. Sandstone geometries may reflect ancient shelf features. Small-scale facies over 305 m may represent large sand waves such as are observed on modern shelves. The elongate northwest-trending lenses are similar in size and geometry to modern marine sand ridges, and the large area of sandstone may have been the site of close-spaced sand ridges.

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