The coal-rich and largely argillaceous mid-A to mid-C Westphalian succession, and the coal-poor and largely arenaceous later C/early D Westphalian succession are very different.
The formation has pebbly sandstones, arkosic sandstones and siltstones, and intercalated thin arenaceous shales.
The arenaceous rocks (arenites) include all those clastic sedimentary rocks whose particle sizes range from 2 to 0.06 mm.
Sediments of arenaceous or sandy texture are characterized by grain sizes which fall between 1/16 and 2 mm.