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1976b) has shown that the Uinta consists of four major tongues (Tu3 through Tus) which are separated stratigraphically by three tongues of Green River Formation: the Dry Fork (at base), Thirteenmile Creek and Black Sulfur (at top). Stratigraphic relationships of these units will be discussed more fully in a later section. Within the immediate Lease area, only three stratigraphic units crop out (Figure 4- 12). Most of the area is underlain by Duncan's (1976a, 1976b) Unit 5 (Tu5) of Uinta Formation. This unit ranges in thickness from 150 to 500 feet and is comprised of brown to buff-colored sandstone and siltstone. Sandstone bodies tend to be channel-form and make prominent outcrops in some of the shallow drainages that transect the Lease. The Tu5 is underlain conformably by the Thirteenmile Creek tongue (TgTu) of Green River Formation. This unit is confined to outcrops along the northern boundary of the Lease near Yellow and Duck Creeks (Figure 4-12) and in Horse Draw. The Thirteenmile Creek tongue ranges in thickness from 10 to 100 feet and consists mainly of light-colored marlstone and marly siltstone and lesser amounts of oil shale and fossiliferous lacustrine limestone. The Thirteenmile Creek tongue is underlain conformably by the Tu4 tongue of the Uinta. This tongue also crops out only along the northern edge of the Lease, and consists of buff to brown cliff-forming, channel-form sandstone and some siltstone and silty marlstone; it ranges in thickness from 0 to 350 feet. The valley bottoms of Yellow and Piceance Creeks and many of their tributaries contain Quaternary alluvium. These deposits range in thickness from 0 to 50 feet and consist of gray to buff-colored sand, silt and gravel. Duncan (1976a) also reports several Pleistocene-aged gravel-covered alluvial terraces along Piceance Creek (not present within the confines of Figure 4-12). Daub & Associates, Inc. Page 4-33 NSI Mine Plan 2010 Rev. Printed: 7/5/2010 Section 4 Geology