In this vicinity, the hogback represents the Dakota Sandstone and Purgatoire Formation, as described below. In ascending order, Purgatoire Formation consists of Lytle Sandstone Member and Glencairn Shale Member. It unconformably overlies the Jurassic Morrison Formation. The Dakota Sandstone and Purgatoire Formation are tilted up in a vertical position, and the hogback appears like remnants of an old castle wall atop a near-continuous ridge. In this vicinity, numerous intrusive sills are present within the rock sequence.
Sills are intrusions parallel or conformable within the surrounding sedimentary strata.
Some of the sills are sufficiently numerous, thick, and resistant to erosion so as to form topographic ridges. Another prominent sill is present at the village of Stonewall. View from near North Lake toward the snow-covered Culebra Range on the western horizon. Cirques are visible on the sides of these peaks. In the foreground, Cretaceous rocks of the foothills are present.