Much more rain falls on Dartmoor than in the surrounding lowlands. As much of the national park is covered in thick layers of peat (decaying vegetation), the rain is usually absorbed quickly and distributed slowly, so the moor is rarely dry. In areas where water accumulates, dangerous bogs or mires can result. Some of these, topped with bright green moss, are known to locals as "feather beds" or "quakers", because they can shift (or 'quake') beneath a person's feet. Quakers result from sphagnum moss growing over the water that accumulates in the hollows in the granite.