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thickness of the group ranges from 2,750 feet to 3,450 feet. The Ile saverde Group in the
<br />region has been dived into two formations. The Iles Formation, the lowermost formation,
<br />consists of marine and continental sequences of sandstone, shale, sandy shale, and coal
<br />beds, with a total thickness around 1,500 feet.
<br />The Tow Creek sandstone member, the basal unit of the Iles Formation, is a massive,
<br />fine-grained, well-sorted sandstone ranging in thickness from 35 feeC to 125 feet, and is
<br />believed to have been deposited in near shore or beach environments. Above the Tow Creek
<br />sandstone is a 1,200-foot interval of sandstone, sandy shale, shale, and coal. The Trout
<br />Creek sandstone member is the uppermost unit of the Iles Formation, and consists of
<br />fine-grained, massive, cliff-forming sandstone about 100 feet thick. The coal zones of
<br />the Iles Formation are distributed throughout the middle and upper portions of [he
<br />formation, and have been assigned to the lower coal group of the Mesaverde Croup.
<br /> The Williams Fork Formation, conformably overlies the Iles Formation and is the upper
<br /> formation of the Mesaverde Group, The Williams Fork ranges in thickness from 1,100 feet
<br /> to about 2,000 feet and includes all of the strata between the top of the Trout Creek
<br /> sandstone member of the Iles Formation and the overlying Lewis shale. The formation is
<br /> comprised of a lower unit, approximately 1,000 feet thick, consisting of shale, thin
<br /> sandstone beds, sandy shale, and coal zones assigned to the middle coal group of [he
<br /> Mesaverde Croup. The middle unit, a massive, cliff forming sandstone varying in thickness
<br />from 100 feet to 200 feet, has been designated the Twenty mile sandstone member, and is
<br />similar in Composition to the Trout Creek sandstone member of the Iles Formation.
<br />Tha upper unit of the Williams Fork Formation lies above the Twenty mile sandstone member
<br />and consists of sequences of sandstone, sandy shale, shale, and coal. This upper unit
<br />exhibits the greatest variation in thickness of any part of the formation. Thfs unit
<br />ranges from 200 feet to B50 feet in thickness, with a westward increase due primarily to a
<br />Change of facies of the strata in [he lower part of the Lewis shale, which overlies [he
<br />Mesaverde Croup. Marine shale strata in the eastern portion of [he region are placed in
<br />[he Lewis shale, While a westward facies change to sandstone, sandy shale, shale, and coal
<br />are plated in [he Williams Fork Formation, The coal :ones deposited s[ra [i graphically
<br />between [he [op of [he Twenty-milc sandstone member and the Lewis shale have been assigned
<br />to the upper coal group of the Mesaverde Group.
<br />• The Lewis shale conformably nvrrlies the Mesaverde Group and, lil.r the 'Nilliams Fork
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