The Trinidad Sandstone was deposited near the shoreline in a marine-to-continental transition during the last retreat of the Cretaceous sea from northern New Mexico and southern Colorado. The Trinidad primarily was deposited in shallow neritic and beach environments. It lies conformably on mudstone of the Pierre Shale and consists successively upward of mudstone with siltstone interbeds, interbedded siltstone and sandstone, and sandstone; inorganic structures include ripple marks, crossbedding, channel-fill structures, parting lineation, and crumpled and contorted bedding. These linear directional structures indicate that the paleoslope of deposition and the direction of sea withdrawal were toward the east-southeast.

Rhizocorallium, Ophiomorpha, Aulichnites, Asterosoma, Teichichnus, Desmograpton, and a yet-unidentified trace similar morphologically to a spiny echinoderm, as well as other tracks and trails, were found in the Trinidad in outcrops from Cimarron northeastward to Raton, New Mexico. These trace fossils indicate that the Trinidad environments dominantly were shallow neritic, littoral, dune, backshore, and in small areas, estuarine.

Amount of carbonaceous debris decreases upward in the stratigraphic section, and the fauna changes correspondingly from scavengers to domicile-building organisms. Dune sediments contain very few trace fossils, but overlying backshore sediments contain a large biota.

The Trinidad is overlain by very carbonaceous to coaly sediments of the Vermejo Formation. The Vermejo was deposited on an alluvial plain, behind the Trinidad beach, and contains only Planolites and shipworm-type traces in carbonaceous rocks.

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