The Tyler Formation in southwestern North Dakota is a regressive barrier-island system dominated by two environments: (1) lagoon and (2) barrier-beach complex. The barrier islands formed along an east-west line in Golden Valley, Billings, and Stark Counties. Thickening eastward (5 to 20 ft), a gradational, coarsening-upward sequence of very fine to medium-grained, well-sorted quartzose sandstone is developed in the Medora, Fryburg, and Green River fields. Where there is good development of a shoreline, massive fine-grained, well-sorted sandstones with discontinuous, wavy carbonaceous laminae (foreshore environment) overlie fine-grained, well-sorted, cross-laminated, and bioturbated sandstones (shoreface environment).
Northward, several offshore sand bars developed in a predominantly shallow restricted sea, typified by medium-gray to grayish-black ripple cross-laminated argillaceous limestones and shales. On the south, in a landward direction, barrier-island sands interfinger with grayish-black carbonaceous lagoonal shales, coal, and mudstone marsh deposits and varicolored mudstone tidal-flat deposits. The sandstones present in the Rocky Ridge vicinity are characteristically medium grained and silty, and exhibit unidirectional cross-stratification. They are interpreted as channel deposits disecting a landward facies of the lagoonal environment.
The upper Tyler Formation is a regressive sequence characterized by anhydritic mudstones, desiccation features, and local chickenwire anhydrites overlying dark-gray fossiliferous, argillaceous limestones and shales.