With the exception of south Pembrokeshire, all parts of the basin are represented by a range of lithologies assigned to the Lower Devonian and to the Upper Devonian, the contact between the two being unconformable and representing the complete omission of any Middle Devonian sequence. The lowermost formations are of upper Silurian age, these being the Downton Castle Sandstone Formation and the overlying Raglan Mudstone Formation except in Pembrokeshire where a more complex series of formations is recognised. In the east of the basin, the top of the Raglan Mudstone is marked by a well-developed calcrete, the Bishop's Frome Limestone. The lowermost Devonian formation is the St Maughans Formation, itself overlain by the Brownstones Formation though with an intervening Senni Formation over much of the area. The Upper Devonian sequence is rather thinner and comprises a series of formations which are more laterally restricted. In the Brecon Beacons, the Plateau Beds Formation is unconformably overlain by the Grey Grits Formation though further east these divisions are replaced by the Quartz Conglomerate Group which is itself subdivided into a variety of different formations.