Geologists have a name for a part of a rock body that has such a relationship to other parts of a rock body. They call it a facies, and define a facies as the general appearance or nature of the one part as compared with the other parts.

A series of modern-day facies could be created by instantly petrifying a part of Florida and entombing its denizens in sediments hardened into an unbroken sheet of rock of the same age. Part of the rock would represent a freshwater facies that might include fossils of freshwater fish, clams, crayfish, and plants in a dark shale that was once mud from a river that fed into a lake. Another nearby facies in the continuous rock layer would include tidal-pool facies fossils such as starfish, tiny saltwater fish, sea anemones, oysters, barnacles, and crabs. Several hundred yards away a new facies might appear, the deep-water one, marked by certain shellfish, corals, larger fish, jellyfish, shark teeth, etc. Each facies, although all are contemporaneous, has a distinct fauna.

Comparable fossil facies exist, such as those in the Silurian coral reefs that are exposed over a wide area from New York to Iowa. A quarry wall where such a reef is being worked may show the top of the ancient coral island with its included broken fossils of mollusks and some arthropods. Farther along in the quarry would be a facies of large, broken chunks of corals, brachiopods, and tough-shelled clams from the edge of the reef, where the surf once pounded. Next to it would be a third facies of the deep water beyond the reef, perhaps a solid limestone with crinoids, bryozoans, and trilobites.

Geologists have their own words to indicate the relationships of rocks and the relative age of the rocks. The amateur will find reference in the professional literature to rocks, for example, as being from the Lower, Middle, or Upper formations of a period, but the period will be divided chronologically into early, middle, and late. The Lower Cambrian rocks will be from the early Cambrian, and will be the oldest of that period. Rocks of a period are called a system and those of an epoch, the next smaller unit, are a series.

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