Bedrock and Surficial Geologic Map of the Red Rock 7.5’ Quadrangle, Beaverhead County, Southwestern Montana

The Red Rock 7.5 minute quadrangle, located in Beaverhead County, southwestern Montana, spans the Red Rock River Valley, an extensional graben formed between the Tendoy mountain front and the western flank of the Blacktail-Snowcrest uplift (Fig. 1). Notable landmarks within the quadrangle include the Clark Canyon Reservoir (Bureau of Reclamation dam number MT00569) located in the northwest area of the quadrangle and Interstate 15 which runs northwest-southeast through the quadrangle. The highest elevations in the map area are located within the Tendoy Mountains and the Red Rock Hills and are underlain by Paleozoic and Cenozoic bedrock. From these points, broad alluvial fans grade down to the Red Rock River Valley. The quadrangle contains about 3,000 ft of relief. Mapping of the Red Rock quadrangle was done at a scale of 1:12,000 and was compiled at a scale of 1:24,000. Field work was completed in the summer of 2005 in collaboration with the mapping of the adjacent Briggs Ranch and Kidd quadrangles (Figs. 1 and 2). This strategy allowed for the comparison of structure and stratigraphy across quadrangle boundaries and provided a regional context for the mapping of each quadrangle. This new mapping complements previous mapping of the Monument Hill quadrangle (Newton and others, 2005), Dixon Mountain quadrangle (Harkins and others, 2004b), Caboose Canyon quadrangle (Harkins and others, 2004a), and Dell quadrangle (Aschoff and Schmitt, 2005) and collectively provides new detailed mapping and analysis of a portion of the Red Rock River Valley from Lima to the Clark Canyon Dam (Figs. 1 and 2). This report includes a map and cross section for the Red Rock quadrangle as well as a discussion of the stratigraphy and structure of the map area