sentences:  The Ordovician period gave rise to a sequence of sedimentary rocks which stretch from Pembrokeshire eastwards through Carmarthenshire up the Vale of Towy and which are intricately intermixed with those of the succeeding period northwards to the Vale of Conwy.In Snowdonia many Ordovician volcanic rocks give rise to a more rugged landscape than elsewhere in the country.Snowdon itself is largely formed of volcanic ash (tuff) with some sedimentary rock and igneous intrusions folded into a syncline.