 The Taffs Well Fault is a continuation of the Daren-ddu fault which is a major NW-SE trending fault in the Coal Measure rocks of the South Wales Coalfield.The Tongwynlais Fault is a normal fault and affects both the Carboniferous Limestone and underlying Devonian Upper Old Red Sandstone.The fault has a downthrow of 85m to the west 

The water is hypothesised to flow largely within the Carboniferous Limestone Group but partly within the Marros Group from sources on the north crop of the coalfield.