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is considered a marker bed in the Mesaverde and has a wide <br /> extent in western Colorado (Full, 1972) . The Rollins sandstone <br /> is considered to be the base of the Mesaverde formation by some , <br /> while others considered it to be the top of the Iles formation <br /> of the Mesaverde group (Full, 1972 ; Collins , 1977) . The <br /> Rollins sandstone is the lowest potential aquifer below the <br /> economic coal seams . <br /> The Rollins sandstone is underlain by a 500 foot sequence of <br /> shale, mudstone, and siltstone (Full, 1972) . This unit is <br /> referred to as the tongue of the Mancos shale in the Iles <br /> formation or as the upper portion of the Mancos shale (Collins, <br /> 1977 ; Full, 1972) . A forty foot thick zone of sandstone under- <br /> lies this 500 foot unit and is referred to as the Cozzette <br /> Sandstone Member. If the overlying 500 foot unit is considered to <br /> be the Mancos shale, this sandstone is considered to be only a <br /> marker bed with the Mancos shale (Full , 1972) . If- the 500 foot <br /> unit is considered to be a tongue of the Mancos shale , the <br /> Cozzette sandstone is considered to be the base of the Iles <br /> formation of the Mesaverde group (Collins , 1977) . The 500 foot <br /> zone of shale, mudstone, and siltstone will serve to retard <br /> downward percolation of ground water from the Rollins sandstone <br /> to the Cozzette sandstone. <br /> The underlying Mancos shale is a massive unit of marine grey <br /> shale over 4000 feet thick in the Coal Basin area (Collins , <br /> 1977) . The Mancos shale is easily eroded and comprises the <br /> 9 <br />