But in practice and in principle, it is common and necessary to identify rocks as a formation, the name of which is usually a compound of a typical location and the rock type, such as "the St. Peter sandstone" or "the Trenton limestone." If the formation consists of more than one type of rock, for instance, if it is of shale and limestone, it may be called by a more general name, such as "the Supai formation," which appears in the walls of the Grand Canyon, or "the Morrison formation," famous for its dinosaur fossils.

Even though in no one place do all rocks of all ages appear as a complete chronicle of the past, in many ways geologists have questioned the mute evidence of the rocks and have arranged it into a coherent story that is concerned more with deeds than with exact dates. Time is reckoned as incidental to the broad picture of earth history. But occasionally geologists are surprised with an incredibly detailed story written in the rocks. Mecca, not the Moslem holy city in Arabia, but a hamlet in western Indiana near the Illinois border, stands where a swampy forest grew 300 million years ago. Close by its forest of horsetails, tree ferns, and scale trees lay an estuary of an inland sea that then covered much of Illinois and parts of Indiana.

Today that ancient Pennsylvanian swamp is known only from a deposit of black shale, but in that deposit scientists from the Field Museum read the story of four years —four years far back in the hundreds of millions that have passed since that time.

Some force, perhaps an earthquake, allowed great quantities of salt water to flood into the swamp, felling the trees. Sharks, primitive fish, and shelled creatures wandered into this new environment, probably attracted by the food washed loose. They flourished until a dry season came. Then the pools shrank, the trapped creatures gasped, fought, mutilated each other, died, and were buried under the thick mat of floating vegetation that turned to black mud and then to shale. The shales show that this happened four times, four seasons of life in the rainy season, death in the dry season. Then the coast settled, the waters grew permanently deeper, and brachiopods, cephalopods, and other invertebrates left their quiet fossil record in the lighter-colored shale that tops the black shale of the death pools. Museum experts under the direction of Dr. Rainer Zangerl, chief curator of geology in the museum, excavated this ancient pond and brought back the remains of huge sharks and thousands of mutilated fish and other fossil creatures from Mecca.

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