Structural Analysis of Hardscrabble Hill, Custer County, Colorado

Hardscrabble Hill is a folded, jointed, and highly faulted sequence of Precambrian to Cenozoic age rocks in Custer County, Colorado. The Wet Mountains Thrust bisects the area and the presence of numerous strike-slip faults complicates the geology locally. Deformational movements in the area range from a few inches on flexural-slip folds to more than a thousand feet along the Wet Mountains Thrust and Billington Fault. Stress analysis of folds, faults, and joints reveal two orientations for the principal stress axes responsible for structures on the hill. These axes are oriented roughly southwest-northeast with a 15° angle between them. No evidence exists of a distinct break in the orogenic process; therefore, it is assumed that orogenic processes were continuous. The 15° change in stress orientations probably was due to changing rates in the orogenic process, or to uplift of small crystalline bodies to the east of Hardscrabble Hill. Initial Laramide tectonic activity involved thrusting and folding, and it was followed by a period of relatively slow orogenic processes. Final stages of orogenic activity were probably more rapid and involved strike-slip faulting and jointing.