The rolling hills of Burren are composed of limestone pavements with criss-crossing cracks known as grikes, leaving rocks called clints. The region supports arctic, Mediterranean and alpine plants side-by-side, due to the unusual environment. The limestones, which date from the Visean stage of the Lower Carboniferous, formed as sediments in a tropical sea approximately 350 million years ago. The strata contain fossil corals, crinoids, sea urchins and ammonites.