Landscape evolution in southeast Wales : evidence from aquifer geometry and surface topography associated with the Ogof Draenen cave system

The evolution of the Ogof Draenen cave system, in south-east Wales, has been profoundly influenced by the geometry of the karst aquifer and its relationship with changes in the surface topography. Using data from within the cave combined with a model of the aquifer geometry based on outcrop data, we have estimated the location and elevation of putative sinks and risings for the system by extrapolating from surveyed conduits in the cave. These data have enabled us to assess the scale and pattern of scarp retreat and valley incision in the valleys of the Usk, Clydach and Lwyd, that together have influenced the development of the cave. From this we can construct a relative chronology for cave development and landscape evolution in the region. Our data show that scarp retreat rates along the west flank of the Usk valley have varied by more than an order of magnitude, which we interpret as the result of locally enhanced erosion in glacial cirques repeatedly occupied and enlarged during successive glacial cycles. This process would have played a key role in breaching the aquiclude, created by the eastward overstep of the Marros Group clastics onto the Cwmyniscoy Mudstone, and thereby allowed the development of major conduits draining further south. In the tributary valleys incision rates were substantially greater in the Clydach valley than in the Lwyd valley, which we attribute to glacial erosion predominating in the north-east-facing Clydach valley and fluvial erosion being dominant in the south-facing Lwyd valley. There is evidence from within Ogof Draenen for a series of southward-draining conduits graded to a succession of palaeoresurgences, each with a vertical separation of 4-5 m, in the upper reaches of the Lwyd valley. We interpret these conduits as an underground proxy for a fluvial terrace staircase and suggest a direct link with glacial-interglacial cycles of surface aggradation and incision in the Lwyd valley. Fluvial incision rates for broadly analogous settings elsewhere suggest perhaps a million years has elapsed since these resurgences were active and implies a significantly greater age for the oldest conduits in Ogof Draenen.