INSTABILITY OF THE EARTH’S CRUST
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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.
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.)