PERMFILE120340

• Permit No: M2007026
• IBM Index Class Name: Permit File
• Doc Date: 5/24/2007
• Doc Name: Adequacy Review Comments
• From: Rio Tinto
• To: DRMS
• Media Type: D
• Archive: No

Colowyo Coal Companp L.P.
Neelsoa Pit
• mining in the Neilson Pit. The entire outcrop is mined and will be reclaimed according to
Exhibit D -Reclamation Plan. The pit does not produce any waste rock and all
aggregate mined is used by Colowyo.
6.3.3.4 Hydrology
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GeoloQV/Topography
The Neilson Pit is located in the Williams Fork Formation of the Cretaceous Mesaverde
Group. The Williams Fork Formation is comprised of discordant beds/units of sandstone,
siltstone and mudstone and coal seams, with an approximate thickness of 1,200 to 1,300
feet in the mine area. Deposited in a deltaic environment, the beds vary in thickness and
lateral extent throughout the Neilson Pit. The sandstones tend to be very fine grained to
fine grained and poorly sorted, with various amounts of silt and clay. For the siltstones
and mudstones, theses units contain various amounts of finer and coarser materials. The
total thickness of the deposit mined is 200 feet thick.
The Williams Fork Formation conformably overlies the Iles Formation. At the top of the
Iles Formation is the Trout Creek Sandstone (TCSS). The TCSS is a massive, white to
light gray, very fine to fine grained, moderately well sorted sandstone with a thickness of
between 50 and 70 feet. This is the only mapped continuous unit in the area of the
Neilson Pit. Beneath the TCSS, the Iles Formation is comprised of sandstones, siltstones
and marine shales.
Two major features, the Collom Syncline and the Danforth Hills Anticline/Wilson Dome,
control the geologic structure in the area of Neilson Pit. The axis of the Collom Syncline,
located approximately 0.5 miles north of the north edge of the reclaimed East Pit for the
Colowyo Coal Mine, trends west-northwest (approximately N60°W) with a slight dip in
the axis to the west-northwest. The Collom Syncline is sub-parallel to the Axial
Anticline on the north and the Danforth Hills Anticline on the south. The Collom
Syncline is asymmetrical, with the north flank of the syncline steeply dipping (20°-40°)
to the south-southwest. The south flank dips to the north-northeast at around 10°t5°.
The Neilson Pit is located on the south flank of the Collom Syncline.
Topographically, the mine is located on a topographic high, bordered on the east and west
by deeply incised valleys. These valleys are Good Spring Creek and Wilson Creek, on
the east and west respectively. The valleys slope from south to north, similar to the
topographic slope at the Neilson Pit.
Hvdroloev
The Neilson Pit is located on both a topographic and structural high. Thus, these highs
cause the mined unit of the Neilson Pit to be above any significant rechazge source, e.g.,
surface water. This is because the bottom of the pit is at an elevation higher than the
elevation of the surface water in the creek. Only when the units are at an elevation lower
than the valleys does any significant recharge occur. Thus, the only source of recharge
for the mined unit in the pit areas is precipitation.
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