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1908 and documented in USGS Bulletin 381 (Washburne, 1910). Field mapping <br />in the ai°ea verified this overthrust feature with a plate angle of 65 <br />degrees to the north - northwest. Coal reserves have been demonstrated <br />to exist in the foot wall adjacent to the over thrust zone. <br />Detailed structural characteristics are illustrated by the Mine Area <br />Geology Map (Map 8), and the Geologic Cross-Sections, (Maps 9 through 11). <br />Strat i Qrap,~y <br />Precambrian Undifferentiated. The oldest rocks outcropping in the <br />Southfield mine area form the west and southwest boundary of the area. <br />Exposed in the eastern flank of the Wet Mountains, this metamorphic <br />complex is predominately a mixture of gneiss and granodiorite. <br />Pierre Shale. Outcropping approximately 1.5 miles east of the mine area <br />is the non-coal bearing Pierre Shale. ,The USGS describes this unit as <br />• an "olive-gray, clayey, silty, and sandy shale that contains bentonite <br />beds and marine fossils throughout" (Scott & Taylor, 1974). The Pierre <br />Shale is characterized by very low hydraulic conductivity and is <br />considered to be an impermeable basal unit relative to the regional <br />groundwater system. At the top of the Pierre is a sequence of thin, <br />interbedded, fine-grained sandstones and mudstones. These sediments are <br />a prodeltaic sequence transgressing on the "Pierre" sea to the east. <br />Total thickness is indicated as 3,900 felt by the USGS. <br />Trinidad Sandstone. Overlying the Pierre Shale and outcropping <br />approximately 1.0 miles east of the mine area are massive sandstone beds <br />of the Trinidad Formation. The basal contact is transitional in nature, <br />indicating a depositional change from the prodeltaic sediments of the <br />upper Pierre to the delta front deposits of the lower Trinidad. <br />Sediment:> of the lower Trinidad are generally very fine to fine grained, <br />clay rich, buff to yellow sandstones with thin mudstone laminae. <br />Overlying these delta front deposits are tributary channel sand deposits <br />M <br />t <br />• <br />