The Lower Palaeozoic rocks of Wales have become, over the past twenty years, a classic example of sedimentary basin fill in a transtensional tectonic setting. The initial mapping work towards this basin model was done piecemeal in a series of Ph.D. theses. Proper illustration and detailing of the model awaited the comprehensive surveys of the Geological Survey. The first phase of this work mapped the east to west transect comprising the Rhayader (179) and Llanilar (178) sheets, published in 1993 and 1994. This mapping showed the way in which sediment was supplied first northwestward from the Midland Platform and then northeastward along the basin axis. It magnificently portrayed how successive turbidite systems were ponded within different fault-bounded transtensional tilt blocks. It illustrated the strong control by these same faults on the structural style due to the later Acadian shortening.

The focus of Geological Survey work then moved southwards, to the transect between the Cardigan sheet (193; published 2003) and the Builth Wells sheet (196; 2004). The missing links in this cross-basin section are now provided by the maps for Llangranog (194) and Lampeter (195). The section provides ample confirmation of the basin model derived further north. The southeastward migration of the southerly-derived Silurian turbidite systems is perhaps even better displayed. The basement-rooted structural lineaments that define the elongate sub-basins are more abundantly evidenced; from the Tywi Lineament in the southeast, over the Central Wales Lineament and the Teifi Anticlinorium, to the Glandyfi Lineament in the northwest. A bonus in these southern areas is certainly the complex Quaternary deposits, straddling the area of influence of both the Irish Sea glacier and the Welsh ice cap. Indeed, it was the land-use and environmental aspects of the Quaternary, rather than of the bedrock geology, that attracted external funding for some of the mapping effort.

Although BGS 1:50000 sheets necessarily have a fairly standard format and style, they do evolve in subtle ways through the years. We have now got used to their being accompanied by the short ‘sheet explanations’ rather than by lengthy memoirs, and these are very adequate for the general user. We are also finding more diverse marginalia to the main map: both the Lampeter and Llangranog sheets include informative 1:150000 digital elevation models of the topography. However, the most momentous change on the new maps is the most subtle; their labelling not as ‘Solid and Drift’ editions but as ‘Bedrock and Superficial Deposits’. We can recognize that the new terminology is more internationally acceptable, whilst admitting some nostalgia for the lost link with the biblical flood.

The new maps deserve a larger volume of sales than they will get in this relatively little-visited region. The Lampeter sheet in particular might make an interesting teaching map, but the abundant ‘drift’ makes ‘solid’ relations a challenge to abstract. Any buyer should note that there is a discount of £3 for buying the map and its sheet explanation together.