Stress-release fractures (hill seams)
Hill seams (also called mud seams, mountain breaks, mountain cracks) are a special type of stress-release fracture or joint. Hill seams form at the surface and tend to parallel the topographic profile of a ridge. They are a common problem in eastern Kentucky coal mines, and can be seen in most highwalls, surface mines, and large roadcuts along state highways and mining roads. Investigations of the U.S. Bureau of Mines indicate that hill seams have been a major cause of roof instability and fatal injuries in underground mines of central Appalachia (Sames and Moebs, 1989).
Hill seams form from stress release. Erosion of a stream valley (or manmade road construction, etc.) removes confining pressure from rock strata in the slope. Loss of confining pressure allows for expansion of the strata outward from the slope in a direction parallel to the valley walls. As cracks open, water can percolate through the openings, which enhances weathering and transports clays and muds into the cracks. Stress-relief joints are most common in steep slopes, under shallow overburden (less than 300 feet), and generally occur within 200 feet of the outcrop (Overbey and others, 1973; Hylbert, 1984; Sames and Moebs, 1989, 1992). Water movement through stress-relief joints is dependent on rock type and tends to move through the hill in a stair-step fashion. Water is perched (moves along the top) of shales and claystones, and percolates through fractures in sandstones (Kipp and Dinger, 1991).
