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EXPLANATION - QUATERNARY and TERTIARY (QTuv)-Volcanic rocks ® TERTIARY (Tu)- (Browns Park and Troublesome Formations) ® TERTIARY (Tl)- Uinta, Green River, and Wasatch Formations TERTIARY and CRETACEOUS (TKl)- (Middle Park Formation) TERTIARY and CRETACEOUS (TKv)- Laramide intrusive rocks ® UPPER CRETACEOUS (Kul)- Mesaverde Group UPPER CRETACEOUS (Ku2)- Mancos Shale, Pierre Shale, and (Colorado Group) ® LOWER CRETACEOUS (Kl)- Dakota Sandstone and Burro Canyon Formation JURASSIC (J)- Morrison Formation and Entrada Sandstone ® LOWER JURASSIC and UPPER TRIASSIC (T)- Glen Canyon Group, Wingate Sandstone, Chinle Formation, (Chinle Formation) PERMIAN and PENNSYLVANIAN (PIP)- (Maroon, Belden, and Eagle Valley Formations) ® MISSISSIPPIAN, DEVONIAN, ORDOVICIAN, and CAMBRIAN ROCKS (HOOF)- (Includes Leadville Limestone) PRECAMBRIAN (pF)- Igneous and metamorphic rocks NOTE-- ST'RATIGRAPHIC UNITS IN PARENTHESES DENOTE UNTI'S LOCATED IN THE SOUTHERN ROCKY MOUNTAINS AREA OF THE BASIN Flgure 3. Generalized bedrock geology of the Upper Colorado River Basin study unit (modified from Schruben and others, 1974)--Continued. Dakota Sandstone composed of sandstones and shales; Mancos Shale; and Mesaverde Group, consisting of sandstone, shale, and coal. The Green River Formation of Tertiary age consists of siltstones, marlstones, sand- stones, limestones, and shales. Rocks of Tertiary and Quaternary age are present in the central parts of the subbasins and consist of predominantly continental sandstones, shales, coals, and locally abundant evaporite minerals. Tertiary volcanic rocks exposed in the study unit partially overlay or intrude many of the rocks described above. The highly variable geologic units of the study unit can be grouped together by the tectonic features present in the basin (fig. 4). These tectonic features represent variations in geology, which also reflect the differences in topography and climate in the study unit. The major tectonic features in the basin include the Gunnison Uplift, Sawatch Uplift, Middle Park Range, Sierra Madre Park Range, White River Uplift, Uncompahgre Uplift, and Piceance Stntctural Basin, which is considered the western extension of the Uinta Basin (Taylor and others, 1986). Geology is similar among the Gunnison Uplift, Sawatch Uplift, Middle Park Range, and Sierra Madre Park Range. In this area of the Southern Rocky Moun- tainsprovince, rocks of gneiss, schists, and granites of Precambrian age crop out and have been intruded by volcanic rocks. This area has been called the Colorado ENVIRONMENTAL SETTING