Complex sentence: The river's course is through upper and middle Devonian rocks, predominantly the Upper Delabole Slates, Trevose Slates and Polzeath Slates that stretch to the coast, although Pentire Head is composed mainly of pillow lavas.
Split into simpler sentences: The river's course is then through upper and middle Devonian rocks, predominantly slates such as Upper Delabole Slates, Trevose Slates and Polzeath Slates. These stretch right to the coast, although Pentire head is composed mainly of pillow lavas.