Isopach map (in color scale) overlain with contours of top Brezzy Hill structure. Proximate analysis, volumetrics and the typical depositional sequence of the Mulky coal and Excello Shale are based on cores from southeastern Kansas. The coal tends to thicken on structural highs. In structural lows the Mulky tends to be carbonaceous shale or high-ash coal rather than a pure coal. The close association with marine carbonate sediments can explain the high-ash and carbonaceous nature of the coal. Peat growth in the lows with admixed marine mud is interpreted to be the cause of the carbonaceous shale or high-ash coals, while low-ash coal develops in mires on highs protected from marine influence.