 The underlying Devonian Old Red Sandstone outcrops over 2 km southeast, just north of the M4 motorway.The superficial geology in the area of the Taff's Well spring comprises an unknown thickness of river alluvium and river terrace deposits.The Taffs Well sub-unit is bound to the south west by the Tongwynlais or Taff's Well Fault, which runs north-south, crossing the River Taff, and passing very close to the Taff's Well Thermal Spring.