Bedrock Geology: Gorseinon is built upon the Grovesend formation. Lithological characteristics are predominantly argillaceous, comprising mudstones and siltstones, with well developed coals; minor lithic ("Pennant") sandstones; locally developed red mudstones in the type area. Description of the lower boundary of this formation is that the base is placed at the base of the Swansea Four-Feet Coal of the Swansea district (equivalent of the Llantwit No.3 Seam in the Pontypridd district and the Mynyddislwyn Seam east of the Taff valley), where it overlies mudstone seatearth at the top of the predominantly arenaceous Swansea Member in the Swansea district and the similar Hughes Member in the east of the coalfield. It is a conformable boundary in the west, but is assumed to be an unconformable one in the east (Woodland et al., 1957; Squirrell and Downing, 1969; Barclay, 1989). Also taken at the base of the laterally correlatable Rudge Coal in the Radstack part of the Somerset Coalfield, the High Coal of the Bristol Coalfield and the Avonmouth No.1 Coal of the Severn Coal Basin. Description of the upper boundary is The Grovesend Formation is the youngest unit found in the South Wales and Forest of Dean coalfields. It is overlain unconformably by sandstones of the Sherwood Sandstone Group in the Newent Coalfield, by mudstones of the Mercia Mudstone Group in the Oxfordshire Coalfield and either the Sherwood Sandstone or Mercia Mudstone groups in the Bristol/Somerset Coalfields.
Swansea Member Formation: Small outcrop of this formation occurs to East of a fault which has a North-South orientation which is in the penllergaer region of Gorseinon. Lithological characteristics are green-grey, lithic arenites ("Pennant sandstones") with thin mudstone/siltstone and seatearth interbeds, and mainly thin coals. Description of the lower boundary of this formation is that the base is placed at the base of the Golden Seam (Swansea Three Feet or Graigola) (Woodland et al., 1957), where the coal rests on mudstone seatearth within a predominantly arenaceous sequence of the Pennant Sandstone Formation. Description of the upper boundary of this formation is that the top is placed at the base of the Wernffraith Seam (correlated with the Mynyddislwyn Seam of the eastern part of the South Wales Coalfield) where the coal overlies mudstone seatearth with a predominantly arenaceous sequence of the Pennant Sandstone Formation, and is overlain conformably by the mudstone-dominated succession of the Grovesend Formation
Drift Geology: The most common drift deposit in Gorseinon are Devension till (boulder clay) glacial deposits. The Afon Lliw is dominated by Alluvium deposits. Tidal flat deposits occur East of Gorseinon near the estuary.
Economic Geology: Several coal mines and shafts occur in South Gorseinon. There are also abundant aggregate quarries in North Gorseinon. Coal seams occur in the Southern and Eastern limits of Gorseinon.
Structural Geology: A large axial plan of an Anticline occurs South of the hospital in Gorseinon with a North West-South East orientation. Four major faults with a North-South orientation in the Gorseinon region.
