as the Welsh geosyncline. Silurian Rocks dating from the Silurian period are, by one measure, the most significant for Wales’ landscape; a greater percentage of the country's land area is directly underlain by rocks of this age than any other. Much of central Wales is formed in Silurian sandstones and mudstones as is the more gentle landscape of central Monmouthshire where the Usk Anticline gives rise to the Usk Inlier. In the north it is the Ludlow age mudstones and sandstones of the Elwy and Nantglyn Formations which form the Clwydian Range, and the Nantglyn Flags and Wenlockian Denbigh Grits which form the Denbigh Moors, Llantysilio Mountain and the Dee Valley around Llangollen. In west Wales, parts of south and central Pembrokeshire around Haverfordwest and Narberth and between Marloes and Daugleddau are formed by Llandovery aged mudstones, sandstones and conglomerates. The Caledonian orogeny A long and complex series of continental collisions known collectively as the Caledonian orogeny began in the Ordovician and continued through the Silurian into Devonian times. The effect in this area was to cause folding and faulting of the existing rock sequence, most particularly within the Welsh Basin, a process which intensified northwards. Parts of the sequence were subjected to low grade metamorphism, the most significant of which, from an economic point of view, would be the Cambrian and Ordovician slates of North Wales. The northeast to southwest 'grain' of much of the country was imparted at this time with a series of major fault zones persisting from that time to the present, some of which, particularly in the northwest, are believed to represent terrane boundaries. The siliciclastic deposits of the succeeding Devonian period represent in large part the rapid attrition of the extensive Caledonian Mountain belt created by the Caledonian collision. Devonian Devonian age rocks are broadly synonymous with the Old Red Sandstone (commonly referred to as ‘the ORS’) though the lowermost ORS is late Silurian in age. The Anglo-Welsh Basin which stretches from the border with England westwards through the Brecon Beacons National Park into Pembrokeshire includes the larger part of this sequence. It is the sandstones and mudstones of the Lower Devonian Brownstones and Senni Formations, sometimes capped by the hard wearing sandstones of the Plateau Beds which form such striking peaks as Pen y Fan and Sugar Loaf and the dramatic scarps of the Black Mountains and Black Mountain. There are restricted occurrences