The Dry Hollow Member of the Frontier Formation in southwestern Wyoming consists of alluvial deposits that accumulated along part of the western margin of the Western Interior Cretaceous seaway. Typically, the lower part of the Dry Hollow Member contains sandstone bodies 4 to 15 m thick that fine upward and possess basal conglomeratic lenses. These units are interpreted as channel deposits of meandering streams, with the conglomerates having formed as channel-lag deposits. Commonly, silicified wood, plant fragments, freshwater gastropods, and intraformational shale clasts are present in these deposits. Medium-scale trough cross-stratification is dominant and indicates a general west to east paleocurrent direction.

The upper part of the Dry Hollow Member is dominated by interbedded shales, coals, and thin (0.2 to 1 m) fine-grained sandstones. The shales contain abundant carbonaceous plant material, silicified wood, and lignitic layers. Root horizons are commonly preserved within large (0.3 to 1 m) concretions that formed in the shales. The interbedded fine-grained sandstones are typically small-scale trough cross-stratified and contain abundant plant material. These sandstones are interpreted as near-channel overbank deposits that accumulated during periods of stream flooding, while the shales and coals represent vertical accretion deposits of interchannel swamps and lowlands that accumulated farther from the stream channels.

Analysis of these deposits suggests that a wet, poorly-drained fluvial lowland dominated by meandering streams and coal swamps existed along the Western Interior Cretaceous seaway during deposition of the bulk of the Dry Hollow Member of the Frontier Formation.

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