The Upper Cretaceous Eagle Sandstone in the Billings area of south-central Montana contains as many as five sandstone units, the lowermost of which is exposed in excellent outcrops along the northern flank of the Big Horn uplift. These exposures, in addition to subsurface control north of the outcrop area, allow determination of the geometry and internal rock characteristics of the sandstone body.

The trend of the lowermost sandstone unit is N. 20-25° W. through Billings. The known length is about 40 miles, the width 20-30 miles, and the maximum thickness 100 feet. The sandstone grades gradually into shale along its lower and lateral boundaries. The contact with the overlying bed is sharp. The dominant structure of the upper part of the lowermost unit is large-scale, low-angle inclined bedding, which dips S. 60° W., approximately normal to the trend of the sandstone. In the lower part the sandstone is mottled and poorly stratified because of burrowing organisms. Grain-size increases in an upward direction. The sandstone is very fine- to fine-grained and well sorted. Chert and rock fragments are common constituents. Glauconite is present throughout the sandstone body, nd muscovite and carbonaceous material are accessories in the lower part. No fossils were observed.

Low-angle inclined bedding, mottled structure, an upward increase in grain-size, and gradational lower and lateral boundaries also characterize Galveston Island, a Recent Gulf of Mexico barrier island. The lowermost sandstone unit of the Eagle is also similar to its Recent counterpart in shape and relationship to regional strand line.

The Eagle barrier island originated as a bar in about 75 feet of water and more than 50 miles offshore. Because the dominant sand-laden currents and waves were on the western side of the bar, the barrier island built toward the mainland. About 20 feet of subsidence occurred during deposition. The subaerial part of the barrier island was removed by erosion during the minor transgression which terminated growth of the feature.