[Illustration: _Camptosaurus_—AN ARNITHISCHIAN PLANT-EATER. (DRAWN BY J. G. GERMAN. COURTESY, AMERICAN MUSEUM OF NATURAL HISTORY.)] HOW WERE THEY PRESERVED? The concentration and burial of dinosaur bones is only the beginning of the fossil story. The combination of circumstances which operated here was a common one and yet fossil quarries are rare. Why? The bones have to be preserved and this seldom happens. The bones that are buried in one flood are frequently unearthed and scattered by the next. Those that are exposed to the weather usually disintegrate completely in a few years. The bones in the Dinosaur Quarry did not. Sometime after they were buried, the organic minerals of the bones were more or less completely replaced by minerals of inorganic origin such as silica. No one knows exactly why or how this happened, but it did. Most geologists think this replacement process occurs when subsurface or ground water containing soluble and colloidal minerals dissolves a molecule of the bone and immediately replaces it with a new mineral. Roughly such a process is like removing red bricks from a house and substituting yellow. When the substitution is complete, the house still has the same dimensions but it is composed of different materials. The replacement was a faithful one, too, because microscopic structure of the original bone was faithfully reproduced by the replacing minerals. [Illustration: ROAD MAP DINOSAUR NATIONAL MONUMENT UTAH - COLORADO High-resolution Map] Following Morrison time, thousands of feet of younger sediments were deposited on the sandbar that contained the dinosaur bones. The whole sequence of sediments was compacted into rock and some bones were crushed and distorted. HOW WERE THEY EXPOSED? After the sediments became rock and the bones had probably been replaced by stone (fossilized), this part of the world, which lay near or below sea level for millions of years, began to rise. Great forces acted upon the earth’s crust. These forces created faults, or fractures, in the rock crust along which movement occurred. And what had once been sea bottom was moved upward and became lofty mountains. This titanic change has been called the Laramide Revolution; it closed the Mesozoic Era with the formation of the Rocky Mountains. [Illustration: _Stegosaurus._ AN ARMORED DINOSAUR OF THE JURASSIC PERIOD. (FROM A DRAWING BY CHARLES R. KNIGHT. COURTESY, AMERICAN MUSEUM OF NATURAL HISTORY.)]