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is generally higher in ash and , in some cases , in sulfur , <br /> than the upper portion. The lower part of the seam is <br /> composed of non-woody plant remains , whereas the upper 6 ' to <br /> 15 ' generally consists of wood-originated macerals . <br /> The A bed coal seam is the lowest in the Coal <br /> Basin strata . The coal rests on the sandstone itself , or on <br /> shales and siltstones of up to 13 ' thickness . The A bed seam <br /> is divided occasionally into an upper and a lower A bed by <br /> a parting of up to 30' thickness . <br /> The B bed consists of the top 6 ' to 8 ' of the <br /> Coal Basin interval . Occasionally , it converges with the A <br /> seam, but for the most part the two seams are separated by <br /> up to 39 ' of rock in the mine plan area . <br /> • The B bed is the principal coal producing <br /> ` seam of Coal Basin and is mined through Nos . 1, 3 , 4 and 5 <br /> mine entries . Approximately 11 million tons of coal have <br /> been mined from the B bed seam since 1956 . <br /> Within the overlieing Williams Fork Forma- <br /> tion, the middle and Upper Sandstone submembers , which occur <br /> within the Bowie Shale Member , form the bases for the diddle <br /> and Placita coal bed seams . The Lower Shale Submember of <br /> the Bowie, a series of mudstone, sandstones and shale of <br /> 370'-400' thickness , separates the Coal Basin Coal Interval <br /> from the Middle Sandstone submember . <br /> The Middle Sandstone is nearly identical to <br /> the Rollins Sandstone. It ranges in thickness from 30' to <br /> 11 <br />