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_lg_ 4) Colorado Yampa Coal Company - Energy Mine No. 3 and the Middle Creek Mine, CMLRD File No. C-036-81 5) Peabody Coal Company - Seneca II Mine, CMLRD Permit No. C-005-80 6) Sun Coal Company -Meadows No. 1 Mine, CMLRD File No. C-029-81 7) Rockcastle Company - Grassy Gap Coal Mine No. 1, CMLRD File No. C-039-81 Mining operations include both surface and underground mining operations. This assessment is divided into three major subsections: Description of the Hydrologic Environment, Probable Hydrologic Consequences of Mining, and Cumulative Hydrologic Impacts of Mining. Surface and ground water are discussed separately in each subsection. Description of the Existing Environment A. Regional Geology The regional geology of the Grassy Gap Mine area is shown in Figure 3. The structure of the area is dominated by two synclinal basins separated by anticlinal structures and associated faulting. The folding and faulting occurred as a result of the uplift of the Park Range to the east and the uplift of the Williams Fork Mountains and White River Plauteau to the south. The Hayden Syncline has a main limb and heading northwest/southeast with a minor limb branching off southeast of the town of Hayden. The Grassy Gap Mine is located on this minor limb and the Seneca II Mine on the eastern flank of the main limb. The Hayden Syncline plunges to the northwest into the Sand Wash Basin. The Twentymile Park the area. There are basin. outlet synclinal basin is the other main structural feature in eight active and reclaimed coal mines which rim this The Twentymile Park Basin is mainly an enclosed basin with its only to the southwest into the Hayden Syncline. The geologic units occurring within the Hayden Syncline and the Twentymile Park Basin range in age from Late Cretaceous to Quaternary (Figure 4). The oldest exposed formation is the Mancos Shale, which was deposited under marine and near-marine conditions. After deposition of the Mancos Shale, sediments of the Iles and Williams Fork Formations of the Mesaverde Group were deposited mostly in terrestrial environments. Following deposition of the Mesaverde Group, sediments of the Lewis Shale were deposited in a predominantly marine environment (BLM, 1976). Younger sedimentary rocks of the Late Cretaceous and Cenozoic periods do not occur in the study area due to erosion or non-deposition. The following description of the geologic units occurring within the upper Yampa Basin is modified from Warner and Dale (1981) and Brogden and Giles (1977).