For decades mine operators in the northern and central Appalachian coal region have struggled to control roof failures occurring in coal seams lying beneath stream valleys. At times the roof in this zone is virtually unsupportable and results in complete collapse. This type of roof failure, known as the notch effect because of the notch it forms in the topography, was once attributed to jointingin the over-lying strata. Because streams often form in areas of weakness in the earth's surface, faults or fractures were thought to be probable causes of this roof problem. Stream valleys in the Appalachian region, however, appear to form at random with little, if any, influence from the underlying bedrock. Their dendritic drainage pattern presumably formed during the normal erosion of relatively weak flat-lying sediments and not from underlying structure. Coal seam continuity beneath the stream valleys further demonstrates the absence of major structural faults. Very small thrust fault structures of quaternary age, which developed as the valleys formed, probably occur beneath most valley floors, however. Also, stress relief joints occur in valley walls and are well- documented. These, too, postdate erosional valley formation and may cause severe roof fall problems, especially in the drift mines of eastern Kentucky, where they are known as hillseams.
Coal, V. 26, No. 3, Mar. 1989, PP. 43-45