sentences:  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.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.