The geology of part of the north crop of the South Wales Coalfield.

An extensive study of the coal mining records, borehole data and underground exposures has provided useful information relating to this area of the Coalfield. The stratigraphic information indicates this area was marginal within the Westphalian basin of deposition. Marine incursions are sometimes multiple and indicate, along with other sedimentary features that the Vale of Neath Disturbance was active throughout the Westphalian as a positive area: there was similar activity around the Taw Valley Disturbance. Newly constructed ideal cyclothems indicate an oscillating delta front from mid Westphalian C onwards so that mesothems replace cyclothems as the basic unit of sedimentation: they also indicate the changing palaeo-environment. Structural information suggests that the present extent of Pennant Sandstone outcron is as it was during the Armorican Orogeny. The Tawe Valley and Vale of Neath Disturbances have probably formed over basement Caledonian faults and were active prior to and throughout the orogeny with possible Neogene movement. An early tensional phase created the normal cross-faults and later compressional deformation took place within the individual fault blocks. The Coalfield was compressed against a northern rigid area resulting in the north-easterly movement of the area between the caledonoid disturbances driving the measures at the margin before it on an imbricate fan and compressional and incompetent structures. The stress-field was resolved and relieved locally by a variety of incompetent structures including the newly described Rotary Faults.