The oldest fossils found in Illinois are shells of marine animals—snails, corals, crinoids, brachiopods, trilobites, pelecypods (clams), cephalopods, bryozoa, arthropods, and others. The youngest fossils are teeth and bones of prehistoric bison, giant beavers, deer, mammoths and mastodons of the “Ice Age,” and snails found in glacial loess.
PLANT FOSSILS (35)
PLANT FOSSILS are the remains of prehistoric plants. Woody structures of plants aid preservation just as hard parts of animals do. Leaves and plants without much woody material generally were well preserved only if they were buried quickly in fine, soft sediment.
The most famous Illinois plant fossils are those from the Mazon Creek area in Grundy and Will Counties of northeastern Illinois. The plant material acted as a nucleus around which iron minerals accumulated to form concretions. Many good fossils—of trunks, branches, leaves, and seeds—are found in coals and in shale directly overlying coals. Descendants of “Coal Measures” plants, such as ferns, mosses, and rushes, are still living today, but they no longer thrive as they did in the warm, moist climate of the Pennsylvanian forests.
Some plants of Pennsylvanian age are petrified, and occasionally such trees or stumps are found. Petrified trees are found also in the upper Mesozoic deposits of southern Illinois. Fossils of “Ice Age” plants closely related to forms living at the present time are occasionally found in peat bogs or scattered throughout glacial deposits.
KEYS FOR IDENTIFICATION OF COMMON ILLINOIS ROCKS AND MINERALS
Two keys, one for minerals and one for rocks, briefly present clues that may aid the collector in identifying rocks and minerals found in Illinois. In outline form, the keys are a guide to some of the easily observable properties that various rocks and minerals display.
The rocks and minerals in the school set of “Typical Rocks and Minerals of Illinois” are included, plus other relatively common ones you might find in Illinois. Because of the great diversity of rocks and minerals in this state, the keys are not conclusive. It is therefore suggested you consult other more complete keys (such as that in Dana’s Manual of Mineralogy) when identifying rocks and minerals that are either from other states or are difficult to identify.
The minerals (p. [30]-35) are arranged in two groups: 1) those with a metallic luster, and 2) those with a nonmetallic luster. Each group is arranged according to increasing hardness. Other characteristics such as color, streak, cleavage, fracture, and composition are listed.