Abstract

The Late Eocene Castle Rock Conglomerate, Tcr on Colorado Geological Survey (CGS) geologic maps, occurs mostly in Douglas and Elbert counties, Colorado. It is the uppermost and youngest Cenozoic unit in the southern Denver Basin and its outcrops occur in a swath trending from Sedalia ∼100 km southeast to Calhan. The unit is well exposed and topographically prominent, forming flat mesas, steep cliffs, and narrow canyons. The conglomerate is a fluvial unit deposited by a 3- to 10-km-wide braided stream system. Large-scale cross-bedding, massive bedding, angular blocks of welded tuff, a variety of other clast lithologies, cut-and-fill structures, fining-upward sequences, fossil logs, and occasional fossilized bones are readily observable. Because the conglomerate is both geologically and scenically striking it has interested geologists since the late 1860s. Because of improved access to the unit over the last 60 years (in Castlewood Canyon State Park and county and municipal open spaces) it has increasingly attracted educators, students, and the public. This article is the second of two parts, and the article as a whole is intended to be a source document for future investigators of the formation. Part 1 (Keller and Morgan, 2024) is a chronology of the description, nomenclature, and mapping of the formation as presented by various investigators over the decades. Part 2 (the present document) covers the formation’s geologic history, age, diagenesis, and possibilities for future research.