Much as snowdrifts are formed, so, in many places, the wind-driven sands are built up into sand hills or so-called "dunes." Dunes are very common in many places, as for example, along our middle Atlantic coast; in Dune Park of northern Indiana; and in the great arid and semiarid regions of the Western States. Where there is a distinctly prevailing direction of wind, the sand is blown to the leeward side from the windward side, and the dunes are caused to migrate in the direction of the wind. The burial and destruction of forests, and the uncovering of the dead trees is not uncommonly caused by migration of sand dunes, all stages of this phenomenon being well exhibited in Dune Park, Indiana. The rate of dune migration is very variable, but study in a number of places has shown a rate of from a few feet to more than 100 feet per year. Arable lands, buildings, and even towns have been encroached upon and buried under drifting sand. An interesting example is a church in the village of Kunzen, on the Baltic seashore which, in a period of sixty years, became completely buried under a dune and then completely uncovered by migration of the dune. Much destruction has been wrought by shifting sands on the Bay of Biscay, where farms and even villages have been overwhelmed. The ruins of the ancient cities of Babylon and Nineveh are buried mostly under wind-blown sand and dust. There is good reason to believe that the climate of central and western Asia is now notably drier than it was a few thousand years ago, and this may help to explain the burial of many old cities and villages there under wind-blown deposits. CHAPTER VII INSTABILITY OF THE EARTH'S CRUST The crust of the earth is unstable. To the modern student of geology the old notion of a "terra firma" is outworn. The idea of an unshakable, immovable earth could never have emanated from the inhabitants of an earthquake country. In general we may recognize two types of crustal movements--slow and sudden. To most people the sudden movements accompanied by earthquakes are more significant and impressive because they are more localized and evident, and often accompanied by destruction of property, or quick, though minor, changes in the landscape. But movements which take place slowly and quietly are often of far greater significance in the interpretation of the profound physical changes which have affected the earth during its millions of years of known history. [Illustration: Fig. 7.--Structure section across the Hudson River Valley near West Point, New York. The shafts and tunnel, 1,200 feet below sea level, in solid rock, show the position of the New York City aqueduct from the Catskills. The Preglacial valley has been submerged and filled with Postglacial sediment to a depth of nearly 800 feet. (Redrawn by the author after Berkey, from New York State Museum Bulletin.)]