COVER. Oblique aerial view to the northwest looking toward the Twin Lakes Reservoir and the snow-capped peaks in the Sawatch Range. Highest peak on the skyline [far right] is Mount Elbert, 4,399 m (14,433 ft). Tree-and sagebrush-covered ridges on the south and north sides of the Twin Lakes reservoir are moraines composed of till of Pinedale, Bull Lake, and pre-Bull Lake age. Arkansas River and US Highway 24 are in foreground. Photograph by Jason P. Briner, University at Buffalo,

and reinterpretation. Mapped surficial deposits and descriptions reflect reinterpretation and reclassification of units and revisions to geographic extent, and lithologic character and map unit representation particularly with respect to mapping of tills in glaciated valleys on the east flanks of the Sangre de Cristo Mountains and Culebra Range by Shroba. Bedrock mapping represents a regional synthesis of geologic data compiled and published over the past quarter century and necessarily includes both simplification and expansion of previously published data with respect to the adopted stratigraphic nomenclature and aerial depiction of map units and faults. Regional and local unit nomenclature is constrained by reference to recently published and previously unpublished 40 Ar/ 39 Ar age determinations on Neogene volcanic deposits-the primary Cenozoic stratigraphic markers in the map area. Map unit descriptions rely heavily on previously published data and are simplified to embody the dominant characteristics of map units particularly where units depicted on larger-scale maps were combined for inclusion in this 1:100,000-scale map. Fault depictions are largely simplified and reflect our interpretation of the most regionally significant and better-exposed structures. Inferred faults that were previously mapped, particularly those buried beneath younger surficial deposits in the San Luis Valley, typically have large locational uncertainties and are poorly dated. Consequently, depiction of these buried faults on this map compilation is minimized (fig. 3; map sheet). Surficial deposits were mapped and compiled by Machette and Shroba (2005-2013). Bedrock deposits were mapped and compiled by Thompson and Fridrich (2005-2013). All authors contributed to the fault compilation. Brandt prepared the digital topographic base map, digital compilation, and GIS database of the geologic map.

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