In the area of Fort Collins, Colorado, the Lower Cretaceous Muddy (or "J") Sandstone can be subdivided into two distinct lithogenetic units. These units, established as a result of detailed description and measurement of surface sections, and recognizable in cores and cuttings from wells in the adjacent subsurface, are herein named the Fort Collins and Horsetooth Members of the Muddy Sandstone.

The lower unit, the Fort Collins Member, is a 12-50-foot-thick white, very fine- to fine-grained clayey sandstone, with sub-horizontal stratification and abundant tracks, trails, and burrows on the bedding planes. It grades downward and southward into the Skull Creek Shale, and is overlain, along a knife-edge contact, by the Horsetooth Member. The Fort Collins Member is interpreted as a near-shore marine sandstone deposited in a late stage of a shoaling begun during Skull Creek deposition.

The upper unit, the Horsetooth Member, is a 1- 40+-foot-thick light gray, fine-grained, well sorted sandstone, with locally abundant carbonized wood fragments. High-angle cross-stratification is common. Ophiomorpha and other burrows are present only in the upper 3 feet; clam molds are present locally at the base. The Horsetooth Member is tentatively interpreted as part of a shoal-water delta (or possibly modified alluvial plain) assemblage, with a central facies consisting mainly of channel sandstones, and southern and northern facies each consisting of channel sands redistributed and reworked by wave action.

Two characteristics of this and other similar ancient models pose somewhat serious interpretive problems: the existence of widespread disconformities separating sandstone members, and the scarcity of mudstone in supposed shoal-water deltaic complexes. Part of the difficulty may be that Recent models are too few in number to account for all the conditions represented in the ancient.