The different types of rock that lie beneath the soil strongly influence the landscape. They determine the vegetation and so the animals that will live in the area. Limestone has fissures (cracks) and is soluble in water, so rivers carve deep, narrow valleys. These rivers then often find a route underground, creating cave systems. Millstone Grit on the other hand is insoluble but porous, so it absorbs water. Water moves through the grits until it meets the less porous shales beneath, creating springs when it reaches the surface again. The shales are friable and easily attacked by frost. They form areas where landslides may occur, as on Mam Tor.