Dunes

Dunes, heaps and ridges of sand,' or less commonly of finely powdered calcareous matter, piled up by the action of winds. They are chiefly found on windward coasts, but also inland, as in the Sahara, Arabia, Utah, and Arizona. They travel in parallel ridges often of considerable height, such as 50 or 60 feet in Norfolk and in Holland, where, however, they sometimes reach

260 feet high over a belt reaching five miles in breadth. They often dam back streams forming pools, salt-marshes, or extensive swamps, as aloiig the coast of Gascony, and, if not arrested, may travel inland from 3 to 24 feet annually, as on the Danish coast. Owing to the destruction of good land thus brought about, they have been planted with the sand sedge ( Carex arenaria) with running stems, or better still, as to the south of Bordeaux, with the cluster pine (Pinus maritima), which is a valuable source of timber and turpentine, besides binding the sands with its roots and breaking the force of the wind. Dunes, 100 to 200 feet high. occur on the south-east of Lake Michigan, and those east of the Caspian have deflected the Oxus into the Sea of Aral. Long parallel lines of high dunes traverse the deserts of central Australia. In Cornwall, the Bahamas, and Bermudas the fine-grained material is largely the comminuted remains of nullipores (q.v.), a group of algae encrusted with carbonate of lime, and this substance is often cemented, by percolating waters into a compact stone.