ABSTRACT

The lithological and petrological characteristics of the South Wales underclays are described. Qualitative X-ray analysis indicates that the underclays of the Upper Coal Measures differ from those of the Lower and Middle Coal Measures in that they generally contain more chlorite and illite and less kaolinite. The South Wales underclays are interpreted as representing allochthonous soils which were deposited by aqueous means before the peat forming flora grew. Their mineral composition is determined mainly by provenance; leaching and weathering were active to only a slight degree. Many of the characteristics of the underclays may be explained by a progressive reduction in the carrying power of the depositing agent. The stratigraphical variation noted in underclay mineralogy is explaine by postulating a relatively drier climate in Upper Coal Measure times together with derivation from a different source.