Mathematical modeling of regional fluid flow in the South Wales foreland basin shows that heat was transferred from internal to peripheral parts of the basin, where very high geothermal gradients and surface heat flow would have developed. In the fluid-discharge areas, temperatures are modeled to have reached 300 °C within the coal measures section and would have generated anthracite, while more internal parts of the basin were cooled by descending fluid flow, and temperatures of only 220 °C resulted, sufficient to form bituminous coal. The modeled thermal regime appears to match the pattern of coal rank observed in the basin. The regional flow probably continued for 1 to 2 m.y. before erosion diminished the topographic gradient driving brine migration in the late Paleozoic.