The depositional environment of the Upper Ordovician Harding Sandstone of central Colorado has been debated since Charles Walcott first described its pteraspidomorph agnathans. Although most previous workers have favored a marine setting for the Harding based on the presence of marine invertebrates and burrows, some have argued for a fluvial or estuarine habitat for these primitive vertebrates. This study presented here is the first regional, comprehensive analysis of the depositional environments and sequence stratigraphy of the Harding Sandstone. The Harding contains deposits from an array of coastal environments, including transition zone, shoreface, bay/lagoon, washover fan, and bayhead delta. Features of these environments suggest that the Harding was deposited on a microtidal coast with low wave energy. Within the study area, two depositional sequences are developed, and each apparently lacks a lowstand systems tract. The Harding contains numerous flooding surfaces, which preclude any attempt to interpret the Harding as a single depositional environment. Flooding surfaces commonly display evidence of stratigraphic condensation, including burrowed firmgrounds, concentrations of pteraspidomorph plates, and dolomitic cementation. Pteraspidomorph remains are associated most commonly with shoreface facies, are uncommon in bay/lagoon facies, and are absent from bayhead-delta facies. Collectively, these occurrences argue for a shallow-marine habitat for these primitive vertebrates.