The geology of Brownhills comprises mainly red clay marl overlying Triassic sandstone and deposits of coal. The town is on several fault lines, the main one being the Vigo Fault, a branch of the larger Eastern Boundary Fault, which runs from Birmingham to Rugeley. On the western side of the fault, in the area of Brownhills Common, the marl is over thinner than on the eastern side, bringing the coal seams significantly closer to the surface. The presence of the faults and the effects of mining mean that subsidence has been a major problem in the area for many years.