Rocks of the Elan Valley Reservoirs

Sunday 28 April 2013 (leader: Colin Humphrey)

Next field trip will be Sunday 28 April. Gather at 10.30 am, car park at Visitor Centre, Elan Valley, Rhayader LD6 5HP (SN 928 647). Driving south on the A470 to Rhayader, turn right at the crossroads and stay on the B4518 for 3½ miles. Bring a packed lunch. Hard hat not essential but advisable if you wish to approach close to rock faces. We aim to return to the visitor centre by around 3 pm (toilets at visitor centre and lunchtime at Claerwen dam). The localities examined lie on or near a single five mile stretch of quiet reservoir road between Caban Coch and Claerwen dams, so we will move mostly by car; the walking is easy.

The roads which wind around the exceptionally beautiful and peaceful reservoirs of the Elan and Claerwen valleys reveal the Cerigwynion Grits which ended the Ordovician here, and the mudstones which began the Silurian. Rising and falling sea-level 440 million years ago produced a range of different types of deep-water sediment deposition. This sandstone and mudstone succession was deposited as a series of gently sloping lobes with slumps, debris flows and a wide range of turbidites. An unusual feature is the record of a very different depositional environment, the Caban Conglomerates, an erosive high energy submarine channel flow which cut through the sandstone/mudstone slope-apron and left its own story in the rock. Local folding (Rhiwnant Anticline) followed by natural erosion and man’s rock cutting and has left different rock facies conveniently juxtaposed.