Highwalls
These are shear cliffs, many times hundreds of feet high that occur during the strip mining process when the ground is removed to uncover a vein of coal. There is danger of falling and the highwalls usually collect deep pools of water at the bottom which is a drowning hazard. The displaced ground must be put back and the area is contoured to its original state.
http://www.dep.state.pa.us/dep/deputate/minres/Districts/abandone.htm