A detailed description is given of the stratotype marking the Namurian-Westphalian boundary in the Carboniferous. Major element analyses are presented for twelve representative samples, including, in upward sequence, underclay, coal, nonmarine shale, and marine shale. The clay mineralogy is described based on powder and smear-mounts of whole-rock samples and clay-sized fractions subjected to standard treatments and cation saturations. Ascending the sequence there is a decrease in the proportion of kaolinite and increases both in the ratio of discrete illite to mixed-layer illite/smectite and in their sum. Chlorite and vermiculite are absent in the underclay but appear higher in the sequence. The upward changes are attributed to progressive flooding associated with a eustatic rise in sea level leading to reworking of mature soils in the flood-plain environment. With time this source decreased in importance compared with the detrital clay input from the primary source area. The underclay in the section has undergone extensive alteration indicative of long subaerial exposure, possibly due to eustatic fall in sea-level.