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• Most of the principal structures in the Nucla Area have undergone a complex tectonic history of <br />recurring differential uplift, subsidence, deposition, erosion, folding, and faulting beginning in early <br />Pennsylvanian time and lasting to the present (Lohman, 1965). The largest of these structures is <br />the 100-mile long Uncompahgre Arch, a northwest trending upwarp whose axis follows the crest <br />of the Uncompahgre Plateau roughly 10 to 15 miles directly north and northeast of the mine site. <br />Otherwell developed anticlines in the area include the Paradox Valley Complex, the Gypsum Valley <br />Anticline, and theSinbadValleyAnticline. These anticlines, in contrast to the metamorphic/igneous <br />core complex found in the Uncompahgre Arch, have intrusive cores of salt and gypsum derived <br />from the Paradox Member of the Hermosa Formation. Far to the west in Utah, Tertiary laccoliths <br />and stocks are evident in the LaSal mountains. <br />Lying between anticlines are several northwest trending synclines which contain the greater part <br />of coal reserves found in the region. Characteristically, these synclines are shallow, broad, <br />structurally simple folds with gently dipping flanks. The Nucla Syncline (San Miguel Syncline), is <br />the largest of these structures occurring in the area. It is approximately 65 miles long. <br />A generalized geologic cross-section showing the mine's setting to adjacent structural features is <br />presented in Figures 2.04.5-1. <br />• In general, the present structural relief of anticlines and synclines found in the region is due <br />primarily to compressive and isostatic forces which produced folding in the late Pennsylvania, late <br />Cretaceous, Eocene and late Pliocene or early Pleistocene times (Cater, 1970). All of the <br />structures show various degrees of deformation along their outer margins in the form of monoclines <br />and high angle faults trending parallel or subparallel to the direction of the axis. A major fault scarp <br />of unknown displacement marks the abrupt southwest edge of the Uncompahgre Uplift, and is <br />shown in Figure 2.04.5-1. Small folds and faults in Quaternary deposits on the crest and salt <br />anticlines may indicate that structural readjustments are still occurring (Cater, 1055). <br />This section has briefly described the regional geology of the Nucla Area as an introduction to the <br />section on site specific geology presented in Section 2.04.6 of the permit application. <br />General Hydrology <br />Regional Hvdroloav. Topography, lithology and structural trends each play a principal role in <br />controlling the quality, quantity and flow characteristics of the regional ground water regime in the <br />• 2.04.5 - 2 <br />