Geologically, the region is defined by coal-bearing layers from the upper Carboniferous period. The coal seams reach the surface in a strip along the River Ruhr and dip downward from the river to the north. Beneath the River Lippe, the coal seams lie at a depth of 600 to 800 metres (2,000 to 2,600 feet). The thickness of the coal layers ranges from one to three metres (three to ten feet). This geological feature played a decisive role in the development of coal mining in the Ruhr.