As a result of the Variscan mountain-building, the Mendip area now comprises at least four anticlinal fold structures, with an east-west trend, each with a core of older Devonian sandstone and Silurian volcanic rocks. The latter are quarried for use in road construction and as a concrete aggregate. The Mendips were considerably higher and steeper 200 to 300 million years ago, and subsequent erosion has resulted in a range of surface features including gorges, dry valleys, screes, swallets and others typical of karst landscapes. These are complemented underground by a large number of caves both beneath the plateau and at the base of the southern escarpment. Wookey Hole is one example of the caves of the area. There are also areas of limestone pavement and other karst features. Dissolution of the limestone produced many of the gorges including Cheddar Gorge and Burrington Combe. Springs are a common feature of the eastern part of the hills, a number of which have associated tufa deposits.