There are many large mountain ranges and high sea cliffs in the region, such as the Derryveagh Mountains, Slieve League, the Cuilcagh Mountains and Ben Bulben. The region is generally underlain by Ordovician and Silurian Shales and Greywackes, laid down 417-495 million years ago. These are found in eastern Cavan and Monaghan. The bedrock of western Cavan, Leitrim and Sligo is composed mostly of Carboniferous limestones that were laid roughly 355 million years ago. However, a 1.5 billion year old narrow strip of metamorphosed gneisses extend across Sligo near the Ox Mountains.