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The Fort Union, Wasatch, Green River and Uinta Formations have a total measured thickness of nearly 8,000 feet in the basin. Each unit displays lateral and vertical variations in lithology and thickness (Figure 4-4). Study of stratigraphic variations has allowed all Tertiary units in the basin to be subdivided into members, except for the Fort Union Formation. Bradley (1931) provided most of the initial subdivisions of the Green River Formation. Subsequent workers, however, have modified many of his subdivisions. Figure 4-5 lists the stratigraphic nomenclature currently in use. 4.2.3.2. Wasatch Formation The Wasatch Formation reaches a maximum thickness of about 5,000 feet in the southern Piceance Creek Basin, where Donnell (1969) has recognized three members: Atwell Gulch (at base), which is comprised of interbedded variegated claystone, sandstone and siltstone; Molina, which is mostly sandstone; and Shire, which consists of variegated siltstone, sandstone and claystone (Figure 4-5). The Wasatch is not subdivided on the Douglas Creek Arch or along the basin's northern margin; however, Johnson (1979a, 1979b) has correlated all three members into the south-central part of the basin. The Shire Member is recognized along the Grand Hogback (Donnell, 1969). 4.2.3.3. Green River Formation Historically, the Green River Formation has been subdivided into four main members: Douglas Creek (at base), Garden Gulch, Parachute Creek and Evacuation Creek (Bradley, 1931). Many of these members, especially the Parachute Creek, have been recognized throughout the Piceance Creek Basin and eastern Uinta Basin (Cashion, 1967). The Evacuation Creek Member is no longer recognized; rocks of this interval are now included in the Uinta Formation (Cashion and Donnell, 1974). Daub & Associates, Inc. Page 4-15 NSI Mine Plan 2010 Rev. Printed: 7/5/2010 Section 4 Geology