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 '^Condit. Cv,wv.-.ji.-.o^5 Illinoi., ...^...c 
 
 ^'^---ind Gilbert Wright Zoolo5.ust, both 
 
 -' ^j~ - • er design my thanks go 
 
 „iM.d fr- : •• •^' ^ ■' •■■■-■- lo 
 
 coopt of these people Iiave been m- 
 
 ■n greatly indebted to them as well as to 
 
 "Ann Livesay 
 
 ptember 19*
 
 STATE OF ILLINOIS 
 Adlai E. Stevenson, Governor 
 
 DEPT. of REGISTRATION & EDUCATION ILLINOIS STATE MUSEUM 
 C. HOBART Engle, Director Thorne Deuel, Museum Director 
 
 STORY OF ILLINOIS SERIES, NO. 7 
 
 r^e Past 
 Speaks to You 
 
 THE STORY OF GEOLOGY IN ILLINOIS 
 
 by 
 Ann Livesay 
 
 Springfield, Illinois 
 1951 
 
 (Printed by authority of the State of Illinois)
 
 SUGGESTED READINGS 
 
 For Those Who Would Like to Learn More About Geology 
 
 Fossils 
 
 Fenton, Carroll Lane. LIFE LONG AGO. New York: Reynal and Hitchcock, 
 
 1937. (Upper Grades) 
 Fenton, Carroll Lane. THE WORLD OF FOSSILS. New York: D. Appleton- 
 
 Century, 1933. (Upper Grades and High School) 
 Hussey, Russell C. HISTORICAL GEOLOGY. New York: McGraw-Hill 
 
 Book Company, 1944. (Upper Grades and High School) 
 Knight, Charles R. BEFORE THE DAWN OF HISTORY. New York: 
 
 Whittlesey House, McGraw-Hill Book Company, 1935. (Upper Grades 
 
 and High School) 
 Romer, Alfred S. MAN AND THE VERTEBRATES. Chicago: University 
 
 of Chicago Press, 1937. (Upper High School) 
 Romer, Alfred S. VERTEBRATE PALEONTOLOGY. Chicago: University 
 
 of Chicago Press, 1945. (Upper High School) 
 Twenhofel, Wm. H. and Shrock, Robert R. INVERTEBRATE PALEONTO- 
 LOGY. New York: McGraw-Hill Book Company, 1935. (Upper High 
 
 School ) 
 
 Minerals and Rocks 
 
 Dana, Edward S. MINERALS AND HOW TO STUDY THEM. Revised 
 
 by Cornelius S. Hurlbut, Jr., New York: J. Wiley and Sons, 1949. (High 
 
 School) 
 Dana, James D. DANA'S MANUAL OF MINERALOGY. Revised by Cornelius 
 
 S. Hurlbut, Jr., New York: J. Wiley and Sons, 15th edition, 1941 (High 
 
 School ) 
 Fenton, Carroll L. and Fenton, Mildred A. THE ROCK BOOK. New York: 
 
 Doubleday, Doran and Co., 1940. (Upper Grades and High School) 
 Loomis, Frederic Brewster. FIELD BOOK OF COMMON ROCKS AND 
 
 MINERALS. New York: G. P. Putnam's Sons, 1923. 
 
 Physical History ot the Earth 
 
 Baker, Russel R. SO THAT'S GEOLOGY. Chicago: The Reilly and Lee Co. 
 
 1942. (Juvenile and Upper Grades") 
 Benson, Allan L. THE STORY OF GEOLOGY. New York: Cosmopolitan 
 
 Book Corporation, 1927. (Upper Grades and High School) 
 Croneis, Carey G. and Krumbein, Wm. C. DOWN TO EARTH. Chicago: 
 
 University of Chicago Press, 1936. (High School) 
 Fenton, Carroll L. ALONG THE HILL. New York: Reynal and Hitchcock, 
 
 1935. (Juvenile and Upper Grades) 
 Fenton, Carroll L. OUR AMAZING EARTH. New York: Doubleday, Doran 
 
 and Company, 1938. (Upper Grades and High School) 
 Longwell, C. R.; Knopf, Adolph; and Flint, Richard F. OUTLINES OF 
 
 PHYSICAL GEOLOGY. New York: J. Wiley and Sons, 1934. (High 
 
 School) 
 Moore, R. C. INTRODUCTION TO HISTORICAL GEOLOGY. New York: 
 
 McGraw-Hill Book Company, 1949. (High School and Adult) 
 
 — 2 —
 
 THE PAST SPEAKS TO YOU 
 
 Imagine the State of Illinois as it appeared "in the beginning" 
 of the immense span of time before life developed here. Upon the 
 land there were no tree, no leafy oaks, no elms, no maples, nor even 
 representatives of the ancient conifers. There was no growth upon 
 the prairies — no red-hued prairie grass, no waving corn field, no rip- 
 pling wheat. There were no houses, no cities, farms or people. For 
 the geological story of Illinois begins with the very young world of 
 long, long ago, before most of the landscape features were shaped or 
 sculptured in their present form. 
 
 Instead of these fertile, rolling prairies, there was once here an 
 inferno of molten reck, glowing and fiery. Drilling records from oil wells 
 show that only a few thousand feet below the ground surface of Illinois 
 today there are remnants of these rocks, now cooled, that were formed 
 in the very early stages of earth history. But that was an almost in- 
 conceivably long time ago, probably more than three billion years ago, 
 when the earth was very young and there was no animal life in what 
 is now Illinois or elsewhere on the face of the earth. 
 
 After a long slow period of cooling which lasted many hundred 
 thousands of years, the molten rock became solidified. Wind and 
 water were at work on the earth surfaces during these centuries, 
 eroding the high places and filling the low spots with their load of 
 sediments, just as these same processes go on today. Slowly the 
 rock surfaces were being sculptured and leveled. With the beginning 
 of the geological time known as the Paleozoic Era or the era of 
 "ancient life" about 550,000,000^ years ago, the picture of events in 
 this area becomes a little more clear. 
 
 The rock record shows us that the area now called Illinois, along 
 with the other Mississippi Valley states, has undergone repeated 
 invasions by shallow seas which spread at different times over much 
 or all of this region. How are we certain that a sea, or more than one 
 sea, ever covered this state? It is the rocks themselves and the 
 fossils- preserved within these rocks that give us the answer. These 
 fossils, which were the plants and animals living during the past, 
 began as primitive, simple kinds which changed through the centuries 
 to different, often larger and more complicated kinds. These fossils 
 have been studied enough so that it is now possible to date the rocks 
 in which these fossils are found. In following the sequence of the 
 development of life throughout the long record in the rocks, begin- 
 ning with the oldest and working upward to the present time, the fos- 
 sils will help to unravel the complex past. Truly, fcscils serve as a 
 key to the long history of the earth. Now we shall turn that key and 
 travel backward in time to the beginning of the Paleozoic, the era 
 of ancient life. 
 
 ' See Appendix I, Kote 1. 
 ^ See Appendix 1, Xote 2. 
 
 — 3
 
 During the first period of the Paleozoic Era, which is called the 
 Cambrian Period, there was only a broad expanse of shallow water 
 extending over the whole area now called Illinois. Very few kinds 
 of life as yet existed in the water and none upon the land. Some of 
 the most important animals were the trilobites, ancient relatives of 
 the crabs and lobsters. The trilobite gets its name from the fact 
 that its body is divided into three parts: the head, abdomen, and 
 tail. Most trilobites had a hard, protective, shell-like armor across 
 the back; and this group of extinct animals, judging from the great 
 numbers of them present in the rocks, was abundant in the ancient 
 seas of which Illinois was a part. 
 
 Other early inhabitants of the Cambrian seas were the shelled 
 animals called brachiopods. These animals could not swim around 
 as the trilobites could but were usually fixed or attached on the sea 
 bottom. Many different kinds of brachiopod shells have been found 
 entombed within the rocks, and even today there are some living 
 brachiopods in the oceans. The only plants that have been recorded 
 from these early Cambrian seas are the algae, primitive forms of 
 plants that have been used as food by sea animals for millions of 
 years. These organisms then, along with some worms, snail-like 
 animals called gastropods, and some primitive sponges, formed the 
 life that was active at the time the Cambrian sandstones and dolo- 
 mites, now underlying our state, were being deposited in the early 
 seas. Just east of Oregon (Ogle Co.) can be seen the only surface 
 exposure of Cambrian rocks in Illinois, but records from oil and water 
 well drillings show us that the rest of the state is underlain by similar 
 rocks buried under younger sediments. Toward the close of this 
 earliest Paleozoic period of time, the land rose slightly and the seas 
 drained slowly off, exposing the land surface to weathering and 
 erosion. 
 
 After a time the land sank again, this time to be covered deeper 
 in the greatest flooding that ever affected our continent. The shore- 
 lines of these seas shifted back and forth with the slow ebb and flow 
 of the marine waters, depositing sediments of the Ordovician Period. 
 Thick deposits of limestone, dolomite, and sandstone"' were being 
 formed in the water. Some of the ancient rock deposits of the 
 Ordovician are well known today. One famous Ordovician rock is 
 called the St. Peter sandstone, from which farmers and city dwellers 
 alike derive pure, fresh water for their needs. It is indeed singular 
 to consider that people today are using water pumped up from a 
 sandstone aquifer or water-bearing rock layer formed along the shore 
 of a widespread sea about 440,000,000 years ago. Some of the scenic 
 features of Illinois, too, have an ancient origin back in these Ordovician 
 rocks; for Starved Rock (LaSalle Co.) is formed of St. Peter sand- 
 stone, and in Apple River Canyon State Park (Jo Daviess Co.) can 
 be found cliffs of still other Ordovician rocks called the Galena and 
 Platteville dolomites. Economic deposits are also known from these 
 Ordovician rocks, including the lead and zinc ores around Galena 
 (Ogle Co.) and some important sub-surface oil pools from Ordovician 
 rocks lying near Dupo (St. Clair Co.) and at other places in the 
 Illinois basin. 
 
 ^ See Appendix I, Note 3. 
 
 — 5
 
 The Saint Peter sandstone as exposed at Starved Rock State Park. (Photo 
 by Hedrick-Blessing, Ltd., Chicago. Courtesy of Division of State Parks.) 
 
 Life was very abundant in the Ordovician seas, and life is 
 thought to have been present only there, since there is no record that 
 either land plants or land animals had yet appeared. In the seas 
 the first group of fishes, small strange-looking creatures covered with 
 bony armor plates over their skeletons, were flicking their tails as 
 they swam about. These first fishes, called Ostracoderms, were of 
 great importance in geologic history because they were the first 
 
 — 6
 
 vertebrates or animals with backbones and thus the forerunners of 
 a host of other vertebrate animals yet to come. Living with the 
 early fishes and being eaten by them as food were the trilobites, which 
 during the Ordovician reached the climax of their development. A 
 great variety of brachiopcds also inhabited the Ordovician seas, and 
 other animal groups like the graptolites, corals, crinoids (sea lilies), 
 and cephalopods (straight or coiled shelled animals of which the 
 chambered nautilus is a surviving descendant) first became abundant 
 in these seas. 
 
 The graptolites of the Ordovician seas are particularly interest- 
 ing because there are no living representatives of this animal group 
 today. The graptolites illustrate a type of animal life called colonial 
 development, since all those that have been found occur in groups 
 or colonies of individuals, all attached by a thread-like connecting 
 structure to a central floating organ. Fragments of graptolite colon- 
 ies are found all over the world wherever the ancient seas covered 
 the land. The only evidences of the existence of these graptolites 
 are thin films of carbon showing the outlines of the former animals. 
 At some places in Illinois, remains of graptolites are found preserved 
 on bits of rock. 
 
 Another extremely varied group of animals, the corals, made an 
 appearance in the Ordovician seas. Some of these corals were soli- 
 tary individuals living alone while others built up coral colonies 
 called reefs, as much as half a mile long. Since most reef-forming 
 corals in the present oceans will live only in shallow waters with 
 temperatures above 68" F., we assume that the corals which lived 
 in these ancient seas must also have required somewhat similar con- 
 ditions. These and many other now fossilized animals were once 
 enjoying the warm, shallow, widespread Ordovician seas. Slow ris- 
 ing of the land, exposing Illinois again to the influence of erosion, 
 brought the Ordovician Period to a close. 
 
 But again there was sinking of the land and another period of 
 flooding by warm, shallow waters in the period of geologic time called 
 the Silurian. This flooding brought about the deposition of shells 
 to form hundreds of feet of limestone and dolomite in the northern 
 part of Illinois, the very important deposits which today are being 
 quarried near Chicago, Joliet and Kankakee, so that rocks formed 
 some 350,000,000 years ago can now be used as soil fertilizers or 
 building stones. 
 
 Marine invertebrate animals continued to be important in the 
 Silurian seas. Coral reefs grew to large size. In the Thornton Quarry 
 at Harvey (Cook Co.) one of these large Silurian reef structures has 
 been exposed to view by the quarrying operation, and recently the 
 finding of important oil pools associated with Silurian reef structures 
 such as those at Marine (Madison Co.) has led geologists to make 
 careful study of reef structure and to search for more oil-bearing 
 Silurian reefs. Other common animals of the Silurian sea were the 
 brachiopods, which showed a marked expansion with many spiny- 
 surfaced forms appearing. Crinoids grew in great profusion and 
 their calcareous plates or tem sections contributed to the limy sedi- 
 ments deposited in the sea waters. Growing on their slender, grace- 
 ful stems, the multi-colored crinoids must have added much beauty
 
 m^: 
 
 Outcrops of Silurian rocks at Mississippi Palisades State Park. (Courtesy of 
 Division of State Parks.) 
 
 to the underwater life. The trilobites, so common earlier, had passed 
 the climax of their reign in the seas, probably partly because they 
 were used as food by the early fishes. But in place of the trilobites 
 there were the eurypterids or "sea-scorpions", small animals from a 
 few inches to a foot in length, which were very common inhabitants 
 of the Silurian seas. With the close of the Silurian Period the sea 
 again retreated and the present Mississippi Valley states area was 
 a low, fiat land area. At the Mississippi Palisades State Park near 
 Savanna (Carroll Co.) may be seen outcrops or surface exposures of 
 Silurian rocks, still bearing fossils of the ancient animal life that once 
 abounded in the seas. 
 
 There were still more sea invasions in store for this region, and 
 only a slight sinking of the land was needed to bring in the seas again. 
 This submergence marked the period of geologic time called the 
 Devonian Period. The seas, swarming with animal life, deposited 
 
 8
 
 more limestones in Illinois, often rich in fossils. This period wit- 
 nessed the evolution of the first known land animals and land plants 
 of all time. This was about 320,000,000 years ago. Corals were 
 again building reef structures and growing to large size, and crinoids 
 were still very common. The brachiopods were at their climax of 
 development, with literally hundreds of variations in shell size and 
 form. Sponges, starfish, and cephalopods all thrived in the Devonian 
 seas. But the greatest diversity and development of all occurred 
 among the fishes, whose teeth, spines, and bony "armour" are found 
 in many Devonian formations. Many sharks swam in the seas and 
 grew to large size. 
 
 Perhaps the most interesting of the Devonian fish is the t5rpe 
 called "lung-fish". This name is used for several groups of fish which 
 among themselves showed two important and fundamental differ- 
 ences from ordinary fish. They were able to breathe air, and they 
 could walk, in a fashion, on land. 
 
 In the latter part of the Silurian and in the Devonian, some areas 
 of North America (and other continents) suffered progressive drying, 
 with streams and lakes becoming reduced to small stagnant pools 
 during part of each year. Ordinary fish of those times, which had 
 invaded streams and lakes from the sea, were killed off in great 
 numbers. The lungfish, however, because of their special abilities, 
 not only survived, but became very numerous. When one pool be- 
 came unliveable, they could make their way to a better one, gulping 
 air and waddling and wriggling awkwardly across the dry land, using 
 their thick muscular fins as legs. The lungfish, thus, were the first 
 vertebrate animals to be able to survive on land. 
 
 In the rocks laid down in this same Devonian Period, the first 
 fossil wood of all time is found, indicating the fact that here are the 
 earliest forests. But here were not found the sturdy, leaf-laden trees 
 of today but strange, slender-trunked, fern-like trees and primitive, 
 hollow-stemmed trees with leaves in whorls. These first trees were 
 the ancestors of the lush forests which later in geologic time were to 
 form the thick coal beds. 
 
 Rocks of Devonian age may be seen today in various parts of 
 Illinois. Perhaps the most spectacular outcrops or exposures of 
 Devonian rocks occur along the Mississippi River from the area 
 around Rock Island (Rock Island Co.) south beyond Keithsburg 
 (Mercer Co.) and in southwest Illinois around Bald Knob and the 
 Pine Hills (Union Co.), and Grand Tower (Jackson Co.) where 
 the Devonian limestones help to form some of the most rugged 
 scenery in the state. Where the Devonian rocks are found buried 
 beneath younger rocks in south central Illinois, these Devonian lime- 
 stones have yielded the largest oil wells in the state at Salem (Marion 
 Co.), and oil has been produced from the Devonian rocks in numer- 
 ous "pools" throughout the southern part of the state. 
 
 Gradually earth history passed from the Devonian into the 
 Mississippian Period and still another interior sea invaded the land 
 to leave its record of limestone and fossils, about 285,000,000 years 
 ago. This Mississippian Period in geologic time has been named 
 from the excellent exposures along the Mississippi River in Illinois
 
 and the adjacent states of Iowa and Missouri. Limestones of Mis- 
 sissippian age outcrop fairly continuously along the western border 
 of Illinois from near Burlington, Iowa, south to Murphysboro (Jack- 
 son Co.). The fact that land and sea fluctuated back and forth 
 often during the Mississippian period is mutely recorded in the rocks 
 themselves. Sandstones and shales are found interbedded with thin 
 limestones of the upper Mississippian times, because rivers were 
 dumping their loads of fine mud and sand continually into the shal- 
 low sea basins where the limestones were forming. The waves along 
 the sea shores sorted the muds and sands carried into the water, 
 depositing the heavier sands near the shores and carrying the lighter 
 mud particles farther out into the water. 
 
 In the deeper water, living conditions were very favorable, and 
 a great variety of animal life existed. Brachiopods of various de- 
 signs and numerous crinoids made up the greater part of the animal 
 life, along with small, single-celled animals called foraminifera, some 
 of which looked like wheat grains. There were some corals, but not 
 of the reef-making types. Bryozoans, strange, lacy-like animal colo- 
 nies resembling seaweeds, were very common in the seas, and tiny 
 snails and clams were abundant in some places. Fishes were also 
 numerous, especially the so-called "shell-crushing" sharks whose 
 jaws were paved with blunt teeth enabling them to crush the shells 
 of clams and brachiopods. 
 
 On the land areas (which apparently did not include what is 
 now Illinois) the amphibians had appeared. These were true land 
 animals, with well developed lungs and legs in their mature stages. 
 Toads and frogs are highly specialized descendants of these Mis- 
 sissippian forms, although the latter looked more like large "mud 
 puppies" or salamanders. 
 
 The amphibians lived in swamps and damp forests made up of 
 large, tree-like plants whose modern descendants are the lowly ground 
 pine, club-moss, and scouring rush or horsetail. These trees, like 
 their living descendants, reproduced by means of spores, a method 
 regarded as simple, primitive and low on the evolutionary scale. In 
 the neighboring state of Missouri many plant fossils of Mississippian 
 age have been found and maybe some will be found in Illinois if 
 one searches enough. 
 
 Mississippian age rocks are important today in the Illinois oil 
 industry, since more oil has come from Mississippian rocks than from 
 those of any other geologic period in such fields as Centralia (Marion 
 Co.), Mattoon (Coles Co.), Olney (Richland Co.), and Salem (Marion 
 Co.). 
 
 At the beginning of the next geologic period, which is named 
 the Pennsylvanian Period for one of the greatest coal-producing 
 states in America, the central United States was a vast lowland 
 stretching out for thousands of miles. There were perhaps some low 
 mountain ridges or at least hills developed at the close of the Mis- 
 sissipian Period, such as the upthrust that occurred in what are 
 now Ford, Champaign, and Douglas counties and which is called 
 the "LaSalle Anticline". This land surface was slowly undergoing 
 erosion while large rivers were helping to remove hundreds of feet 
 
 — 10 —
 
 Forests such as this flourished in Illinois about 250 million years ago when 
 giant dragonflies and huge cockroaches lived among the coal-forming plants. 
 (Miniature scene by Paul Marchand in Museumobile. Photograph by C. Hodge.) 
 
 of older sediments. Later in the Pennsylvanian Period there was 
 sinking of the land which resulted in coarse sands and gravelly de- 
 posits being carried in to fill up the depressions which erosion had 
 created. After a time the Mississippi Valley states became more or 
 less stable, and there was developed here over the low, flat expanse 
 of land a vast series of swamps where vegetation of the primitive 
 plant types was very abundant. In these swamps, the decaying 
 vegetation accumulated as thick beds of peat. Sometimes sand and 
 mud were washed into the swamps, burying the peat beds. And 
 sometimes marine waters invaded the low-lying areas, leaving the 
 record of their presence in the form of thin limestone layers contain- 
 ing typical marine fossils. Then the land would rise again and the 
 swamps would return to build up further accumulations of peat. 
 As a result of these continued cycles of deposition, there was built 
 up in Illinois a thick series of limestones, shales, sandstones, and 
 coals from which we now obtain some of the world's finest bituminous 
 coal, formed in the Pennsylvanian Period over 200,000,000 years 
 ago. Today these Pennsylvanian rocks lie at and near the surface 
 of about three-fourths of Illinois. Pennsylvanian clays and shales 
 are now used in the manufacture of brick, and limestones are used 
 in making portland cement. Illinois coal No. 6 or the "Herrin Coal", 
 which is 6 to 11 feet thick over many counties in Illinois, is one of 
 the really great coal beds of this country. 
 
 As might be expected in a region where mild, moist climates pre- 
 vailed over large areas, the animals and plants of the Pennsylvanian 
 Period were very abundant. The largest insects of all time winged 
 
 11 
 
 iwasr*
 
 their way through the swamp forests. There were spiders, scorpions, 
 and centipedes scurrying around; and huge dragonflies with wing- 
 spreads of 29 inches. Large cockroaches were so common that this 
 period of geologic time is sometimes called the "Age of Cockroaches". 
 There were land snails, fresh-water clams in the rivers and lakes, 
 and over fifty different kinds of amphibians. These amphibians were 
 all small salamander-like animals. 
 
 Near the end of the Pennsylvanian Period is found evidence of 
 another great event in geologic time — the appearance of the reptiles. 
 Reptiles are more advanced animals than amphibians because they 
 are completely independent of water, while the amphibians' eggs 
 and young cannot live out of water. In contrast, reptile eggs, cov- 
 ered by strong membranes, are laid on land; and the young are 
 completely adapted to land life from the moment they come out of 
 the egg. The reptiles, beginning with small sizes in the Pennsyl- 
 vanian Period, were destined to become later some of the largest 
 and most terrifying animals. The reptiles, amphibia, and other land 
 animals lived in widespread swamp forests made up of spore-bearing 
 trees like those of the Mississippian as well as some more advanced 
 types. A particularly abundant newer kind of tree is called the 
 "seed-fern". These plants grew to large size, bore leaves that re- 
 semble closely the spreading, graceful fronds of ferns, but instead of 
 spores, had large, hard-shelled seeds enclosed often in a cup-shaped 
 structure. These trees are some of the earliest representatives of 
 the seed plants which are today the abundant, widespread, and com- 
 mon plants. The seed-ferns did not have flowers, but are more 
 closely related to living conifers such as pine, cedar, and firs. These 
 forests contributed to the great coal beds of Illinois, and in the region 
 around Braidwood (Will Co.) and Coal City (Grundy Co.), many 
 nodules containing leaves, seeds and other parts of these trees can 
 be found in the tailings of the strip mines. 
 
 In the Pennsylvanian seas lived numerous brachiopods, bryozoa, 
 crinoids, corals, clams, fusulines and a few trilobites, all of which, 
 and many others, have left their shells in Pennsylvanian marine rocks. 
 
 After the deposition of the Pennsylvanian rocks, the region of 
 the Ozark Mountains of Missouri was uplifted slowly to form a low 
 mountain range which extended across southern Illinois. At the 
 present time, these Illinois Ozarks are by no means as high as they 
 were originally. Doubtless thousands of feet of sediments have been 
 eroded from these hills. As this uplift of the land took place, there 
 were formed numerous cracks and fissures in the rocks, which were 
 later filled by molten igneous (or "fire") rocks from below. This 
 now-cooled igneous rock can be seen at the surface in a few places 
 in southern Illinois. This combination of igneous rocks being pushed 
 up and the formation of cracks and fissures is believed to have been 
 responsible for the accumulation of the famous fluorspar deposits 
 of Pope and Hardin counties. When the beautifully colored blue 
 and purple fluorite crystals were first found in southern Illinois, 
 their value was not known and fluorite was judged worthless. But 
 now Illinois, along with Kentucky, ranks as the world's leading pro- 
 ducer of this extremely critical natural resource; for now fluorite 
 is essential as a flux in steel production, in insecticides, in hydro- 
 
 — 12 —
 
 Fluorite, a hard, lustrous mineral, occurs in colorless, green, blue, purple, 
 yellow, brown or black crystals, usually in cubes. (Photograph by Charles Hodge.) 
 
 fluoric acid, in the extraction of metallic aluminum from its ores, 
 and in countless other industrial processes. Thus the events that 
 occurred near the close of the Paleozoic Era in geologic time have a 
 direct bearing on our life today. This elevation of the Illinois Ozarks 
 occurred at the same time as the original development of the Ap- 
 palachian Mountains about 225,000,000 years ago and marked the 
 withdrawal of the seas from most of the central United States. 
 
 We shall never know exactly what events took place during the 
 next few million years in Illinois because this record is forever lost 
 to us. There are no rocks in Illinois representative of the Permian, 
 the Triassic, or the Jurassic Periods of geologic time. Since there 
 are no rocks of these ages, there are likewise no fossils. It is pos- 
 sible, however, to learn much from adjacent areas where rocks often 
 rich in fossils are found. There is no reason to believe that condi- 
 tions in the Illinois area during these three periods were greatly 
 different from those in adjacent areas, for which we have evidence 
 both in rocks and fossils. 
 
 Judging from the fact that there are no rocks anjrwhere in 
 Illinois of Triassic or Jurassic age, geologists believe that this state 
 was constantly above sea level for this immensely long period of 
 time. Upon this land surface there probably roamed the genera- 
 tions of huge and terrible dinosaurs and countless other reptiles long 
 since extinct. Although their bones are not found here in the state, 
 other deposits in America bear mute testimony of the size and 
 strength of these long-ago monsters. The tracks and remains of the 
 first dinosaurs are found in parts of the United States where the 
 Triassic rocks were deposited over 170,000,000 years ago. At first 
 
 — 13 —
 
 the dinosaurs were small, scarcely 3 or 4 feet long. Their bones 
 were small and slender, their footprints tiny and shallow. But by 
 the middle of the Triassic Period, the dinosaurs were larger in num- 
 bers and in size; and they left large, deep tracks where they ponder- 
 ously moved about. While other reptiles sprawled along the ground, 
 these early dinosaurs moved with a running locomotion off the 
 ground, supported by the hind legs. This gave the dinosaurs an 
 advantage over other reptiles; they could catch food faster by being 
 able to run. In the Triassic Period, some of the other reptiles began 
 to return to sea life, probably partly to escape from the dinosaurs 
 on land and partly to get more food from this abundant source, 
 the sea. There were two main groups of these marine reptiles, one 
 called the ichthyosaurs and the other the plesiosaurs. The ichthyo- 
 saurs were fish-like reptiles, fast swimmers that reached a length of 
 30 feet. The plesiosaurs were broad, short-tailed creatures resembl- 
 ing huge turtles. They paddled around clumsily but had long necks 
 which could dart out suddenly to catch unwary animals nearby. 
 In these Triassic seas, which probably never reached Illinois, were 
 reef-building corals, clams, cephalopods, coiled and beautifully 
 shelled gastropods, and a few pelecypods. Nearly all of these in- 
 vertebrates, along with the fishes, served as food for the hungry 
 marine reptiles. On the land there appeared just at the close of the 
 Triassic Period of geologic time the very first mammals, extremely 
 important animals to us who belong to the same tribe. But few of 
 us would probably with pride claim kin to these first diminutive, 
 insignificant creatures! The early mammals, no bigger than a rat or 
 mouse, were equipped with sharp teeth, and already showed a pro- 
 portional increase in brain size over that of the reptiles. 
 
 Thousands of centuries passed and the land was still above the 
 sea in the Mississippi Valley states. There are no rocks, no fossils 
 to record the passage of the millions of years of the Jurassic Period 
 here. Was Illinois a highland or a lowland? No one knows, but 
 geologists think this area was considerably higher then than now. 
 In the Jurassic Period, as in the Triassic, there is fossil evidence from 
 other regions which tends to show that life was abundant in Illinois 
 during this time. There were about one thousand different insect 
 species, including dragon-flies, grasshoppers, beetles, cockroaches, 
 termites, ants, flies, and moths in this Jurassic world some 145,000,000 
 years ago. By this time the dinosaurs had reached their grestest 
 size, some of them as much as 65 to 80 feet long! There is one kind 
 of dinosaur on record as weighing 10 tons but whose brain weighed 
 only 2V2 ounces. All the dinosaurs were noted for their extremely 
 small brains, and indeed they must have been ponderously stupid 
 animals for all their huge size. The marine reptiles were at the 
 zenith of their development in the Jurassic seas, preying on fish and 
 marine invertebrates for food. It was during this period of geologic 
 time that the first birds evolved from the reptiles. These first birds 
 would seem strange indeed when compared to a modern bird. Jaws 
 set with rows of small teeth, three claws at each wing tip, a long, 
 peculiarly shaped tail: all these features were evidences of the rep- 
 tilian origin of the first bird, known to scientists as Archeopteryx. 
 
 — 14 —
 
 „i. 
 
 
 if 
 
 This Jurassic landscape shows Archeopterix (the first birds) in the center, 
 the strange pterodactyls or flying reptiles in the air and on the tree trunk, and 
 at the left the tiny dinosaurs which were ancestral to the later and larger horde 
 of dinosaurs. (From a painting by Charles R. Nnight. Courtesy of the Chicago 
 Natural History Museum.) 
 
 These land animals lived in a wide variety of environments. 
 There were extensive deserts in some places, broad, rich river valleys 
 thickly clothed with plants in others, and widespread swamps, with 
 dense forests in others. Most of the plants of the Triassic and 
 Jurassic were similar to some that are living today. There were 
 pines, firs, redwoods and ginkgos almost like living ones, and in 
 addition a large number of plants called cycadeoids somewhat like 
 the living sago-palm. These plants had thick, rough trunks crowned 
 with a mass of large, palm-like leaves, and bore flower-like structures 
 either on the trunks or in the joints of the branches. They were not 
 true flowering plants, however, but belonged to the great plant group 
 of gymnosperms along with the conifers. 
 
 Compared to the huge dinosaurs, the mammals were still small 
 and inconspicuous in size, but the mammals were spreading all over 
 the world into new and diverse environments. In the seas animal 
 life was abundant and was beginning to resemble modern animals 
 in general features. There were coral reefs, pelecypods, lobsters and 
 shrimp-like creatures, crinoids, sponges, and exquisitely fluted cepha- 
 lopods possessing delicate pearly shells. 
 
 16 —
 
 With animal and plant life developing on a grand scale, geo- 
 logic time passed into the Cretaceous Period. Early in this period, 
 there was recorded in the rocks another sea invasion, spreading over 
 this continent from the Gulf of Mexico and reaching just to the 
 southern tip of Illinois in its northward advance, where thin deposits 
 of sand, clay and gravel record its presence. Elsewhere in the United 
 States, very thick beds of mud and silt were being laid down in 
 shallow seas; but in the rest of Illinois, low lands were still just above 
 sea level. On these lowlands there appeared angiosperms, the true 
 flowering plants. Some of these early trees were the magnolia, 
 poplar, fig and sassafras. In most areas the angiosperms quickly 
 replaced the gymnosperms, which lost their supremacy and were 
 represented by only a small number of conifers in temperate and 
 cold regions. At this time the sequoia tree, now limited to 
 small areas in California, was widely distributed over the 
 Northern hemisphere. By the middle of the Cretaceous Period, the 
 forests were almost like those of today with maple, oak, beech, wal- 
 nut, and other trees common in the forests of today. The spread of 
 this highly specialized group of plants during Cretaceous tim.e some 
 90,000,000 years ago, was one of the most important milestones in 
 the development of life on earth, since flowering plants provide nearly 
 the entire food supply for the herbivorous mammals that now are 
 so abundant on the earth. 
 
 Among the animals, the Cretaceous was a time of dying out for 
 some and a time of spreading out and growth in other groups. This 
 period has been appropriately called "the time of the great dying". 
 The dinosaurs reached their climax and died; the pterodactyls or 
 flying reptiles, the ichthyosaurs and plesiosaurs, and most of the 
 coiled cephalopods disappeared. Why did all these varied forms of 
 animal life come to an end at about the same time? What were 
 the causes, world-wide and relatively sudden, that led to such appar- 
 ent mass extinction? Those who have studied these problems be- 
 lieve that several causes worked together to bring an end to many 
 of the Cretaceous forms of life. There was great restriction of the 
 seas at the end of this geologic era, disappearance of the marshy 
 lowlands inhabited by many kinds of dinosaurs, a drop in tempera- 
 ture in some areas due to the rising of the Rocky Mountain ranges, 
 and changing types of vegetation. Those groups of animals and 
 plants which were unable to change and to adapt themselves to 
 these changing environmental conditions were forced to yield and 
 give way to those plants and animals which could and did make the 
 necessary adjustments to fit the changing conditions. And so many 
 of the Cretaceous animals and plants died out; they died by the 
 thousands, leaving their bones, shells, leaves, stems, teeth, and other 
 parts as evidence of once powerful hordes that could not change and 
 for that reason could not live on in a changing world. 
 
 But all life was not doomed to destruction at this time. There 
 were encouraging signs among some groups of animals. As the 
 dinosaurs disappeared, the pouch-bearing mammals like the kangaroo 
 and the primitive placental mammals called insectivores began to 
 develop. In the seas the clams and the gastropods were holding their 
 
 — 17 —
 
 In the Cretaceous Period, the dinosaurs reached the climax of their develop- 
 ment. Two typical beasts of those days were Tyrannosaurus (right) which was a 
 flesh-eater with powerful jaws, and Triceratops (left), a vegetable-eater which 
 was not aggressive despite its long, sharp horns and armored frill. (Miniature 
 scene by Paul Marchand in Museumobile. Photograph by Charles Hodge.) 
 
 own amid the changing conditions, and crabs and oysters were corn- 
 men. And so with the disappearance of some animal and plant 
 groups, the appearance of others, the building up of some of the 
 earth's great mountain chains, and the disappearance of the seas 
 once more from Illinois, the Cretaceous Period of geologic time drew 
 to an impressive close, bringing us still nearer to the present. 
 
 As the earth passed into the latest period of geologic time, known 
 as the Cenozoic Era, vast changes were apparent in animal life. The 
 land surfaces exposed above sea level were larger than ever before, 
 and over these lands roamed thousands of odd-looking mammals in 
 this world of 60 million years ago. Most of these early mammals of 
 the Paleocene Epoch, the epoch of "ancient recent life", were small 
 in size, had small brains, long, narrow heads, and long snouts. These 
 early mammals had five functional toes on each foot and walked on 
 the sole of the foot. At the beginning of the Eocene Epoch, the 
 "dawn of the recent", the ancestors of most of the modern forms of 
 animals made what appeared to be a sudden, abrupt entrance upon 
 the scene. It is believed that many of these animals had been de- 
 veloping in Asia and at the beginning of the Eocene had migrated 
 into North America along a land bridge across the Bering Straits. 
 There were tiny horses, small rhinoceroses, camels, monkeys, and 
 rodents. All of these were small and had small brain capacities. 
 
 There are in extreme southern Illinois some thin deposits of 
 unconsolidated sands, clays and gravels which were laid down in 
 
 — 18
 
 the northernmost extension of the Gulf of Mexico which once ex- 
 tended far up the Mississippi Valley. One of these deposits, which 
 is called the Porters Creek formation, has become important com- 
 mercially in Pulaski County, since it contains fuller's earth, which 
 is very useful in removing grease from cloth and in deodorizing and 
 decoloring oils and fats, due to its power to absorb impurities. These 
 Paleocene and Eocene deposits of the Gulf of Mexico "embayment" 
 are thin and represent shore-line deposits in Illinois but are much 
 thicker and more important in states farther south. 
 
 By the time of the next geologic epoch, the Oligocene, there 
 were beavers, squirrels, rabbits, mice, dogs, biting and stabbing cats, 
 oreodonts (extinct pig-like animals), several kinds of rhinoceroses, 
 and many other animals. Life was becoming more and more like 
 that of the present. No rocks or sediments of Oligocene age are 
 known in Illinois. 
 
 The next epoch, the Miocene, has been called the "Golden Age" 
 of the mammals. The spectacular spread of mammals at this time 
 was due in large part to the spread of the prairie grasses upon which 
 the herbivorous mammals fed which in turn were the food supply 
 for carnivorous mammals. Increase in size among many groups of 
 herbivores at this time and changes in their tooth structure are both 
 due to the animals feeding upon the coarse prairie grasses. Some 
 of the prominent Miocene animals were horses, camels, rhinoceroses, 
 giant pigs, wolf-like dogs and cats. Again there are no rocks of this 
 age found in Illinois, but it is thought that all of these animals 
 probably roamed over Illinois at this time, feeding upon the prairies 
 that must have been here. 
 
 During the next geologic epoch, the Pliocene, mammals grew in 
 size and further migrations from Asia brought in the mastodons, now 
 extinct, which were elephant-like tusked animals. It was during 
 Pliocene time that the last thin layer of gravels and sands of the 
 Gulf of Mexico embayment were deposited in southern Illinois. 
 Throughout all of the geologic periods from Paleocene through Plio- 
 cene, from 60,000,000 years ago to 1,000.000 years ago, there were 
 four major trends in mammalian development. These four trends 
 were: increase in brain capacity, increase in size, specialization of 
 the teeth according to the food used, and specialization of the feet 
 according to the environment of the particular animal. 
 
 The horse has often been used as an example of the progressive 
 evolution within a single animal group. The ancestor of the known 
 horses must have had five toes on each foot and must have been an 
 extremely small animal, but no remains of this ancestral horse have 
 yet been discovered. The first type of horse whose skeleton has been 
 found and studied is from the Eocene. This early horse had three 
 toes on each hind foot and four toes on each front foot. It was a 
 small, slender animal not over a foot in height and is known as 
 Eohippus ("the 'dawn horse"). By the Oligocene, the horses were 
 about the size of large sheep and had three toes on each foot. All of 
 these toes touched the ground and helped to bear the weight. This 
 type of horse has been given the name Mesohippus. By Miocene 
 time, the horses had grown to the size of small ponies. Constant 
 running, probably to escape from enemies, had caused the animals 
 
 — 19 —
 
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 :(Eocene) ^Q^ohtppus 
 
 iOHgocene) ivfiohipp*^^ 
 
 (Miocene 
 
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 pfiohtppys 
 (Pl^cene) Eqoos 
 
 /Pleis 
 
 tocene) 
 
 Progressive evolution of the horse showing changes in foot and skull structure. 
 (Courtesy of Peabody Museum of Yale University.) 
 
 to rise up more and more on their toes to gain more speed, just as 
 human runners rise up on their toes to gain speed in a race. This 
 Miocene horse, although it still had three toes, used only the middle 
 toe in running. Consequently the two side toes dangled along the 
 sides of the foot and didn't reach the ground. This Miocene horse, 
 called Merychippus, also had longer jaws and stronger teeth to chew 
 up the coarse grasses than the smaller horses had. By Pliocene 
 time, the horses had only one functional toe in use on each foot and 
 as a result were among the fastest running animals. The side toes 
 had been so much reduced that they were represented only by a 
 pair of splint bones lying along the side of the leg. So when a modern 
 horse has trouble with a splint bone it is really one of his lost toes 
 that is bothering him. From the pony-sized, fleet-footed horses of 
 the Pliocene it was only a short step to the horses of today, which 
 still walk and run with their entire weight borne by one toe on each 
 foot. 
 
 Along with the evolution of the mammals during the epochs of 
 the Tertiary Period, there were also changes in the plants. So far 
 as the fossil record shows, this was less a structural evolution than 
 it was changing geographic distribution. In the early Tertiary 
 (Eocene and Oligocene) forests apparently stretched almost con- 
 tinuously across North America from coast to coast. Also, the dif- 
 ferent kinds of forests lived a long way north of their present posi- 
 tions. In Alaska, for instance, we find in Eocene rocks the remains 
 of plants very like those that now occupy temperate latitudes. In 
 British Columbia and Washington, there lived plants related to those 
 
 21 —
 
 Part of the terminal moraine deposited by the melting Wisconsin glacier near 
 
 of the Gulf States today. And in California and Oregon, as well as 
 in the Gulf States, lived plants now found in Central America, north- 
 ern South America and the East Indies. 
 
 Two important changes took place in this distribution during 
 the remainder of the Tertiary. First, the forests moved southward, 
 attaining their present latitudes in the late Pliocene. Second, as a 
 result of the uplift of the western mountains and the withdrawal of 
 the sea from the Mississippi embayment, extensive dry regions and 
 desert areas developed in the plains region. As a consequence, the 
 eastern and western forests were divided by the development of 
 wide-spread grasslands. It was in these grasslands that the horse 
 and many other fast running, grass-eating mammals developed. 
 
 Leaving the story of Tertiary evolution, a part of which doubt- 
 less took place in Illinois, there comes the last great episode in the 
 shaping of the landscape, the Pleistocene Period. Again it may 
 seem difficult to believe the events which geologists have shown 
 happened here within the last million years. Where fertile prairies 
 and shade trees now grow, where thriving cities now hum with 
 activity, there were glaciers moving slowly over the surface of the 
 ground, grinding, crunching, pushing forward, restricting plant and 
 animal life, driving them southward to escape the cold and the ice. 
 Not just once but at least four different times within this Pleistocene 
 Period or Ice Age, the ice sheets moved southward to cover parts 
 of Illinois. The ice advance which covered nearly the whole of this 
 
 22—
 
 Shelby ville, Illinois, on Illinois Highway 16. (Photograph by Charles Hodge.) 
 
 State, reaching nearly as far south as Cairo (Alexander Co.), is called 
 the Illinoian glacial stage and occurred some 250,000 years ago. 
 The last period of glaciation to reach Illinois was only about 10,000 
 years ago in the Wisconsin glacial stage. 
 
 These ice advances over Illinois have left many evidences of 
 their presence in the form of various kinds of glacial deposits which 
 have been given different names depending upon their sizes, shapes, 
 and the types of materials contained. Some of these glacial deposits 
 are known as moraines, some as kames, drumlins, eskers and glacial 
 swamp and lake deposits. On the surfaces of some rocks may be 
 seen the grooves and scratches made by other rocks as they were 
 pushed and ground along by the ice. Along with the gravels and 
 coarse sands which the glacial waters deposited as the ice melted, 
 these glaciers left in Illinois a priceless heritage — the materials which 
 through later soil-forming processes have given us our fertile soil. 
 Scraped up and carried along by the glaciers from Canada and Min- 
 nesota, this rich "parent" soil material was deposited over much of 
 Illinois and Wisconsin as the glaciers melted. The present soil 
 mantle, taken as a whole, is much more valuable than the few small 
 diamonds, gold nuggets, and pieces of copper which the glaciers also 
 carried down with them from the North. These same glaciers formed 
 the Great Lakes by scooping out their basins through the eroding 
 action of the moving ice. The water courses of the Illinois, Ohio, 
 Mississippi and Wabash Rivers were also changed or reshaped dur- 
 
 — 23 —
 
 
 *'***' ^ 
 
 «**W 
 
 ^.^ "«ra 
 
 During the Pleistocene Period or Ice Age when the glaciers receded and 
 tundra growth furnished a food supply, the huge woolly mammoths with their 
 great curved tusks roamed over Illinois, along with the woolly rhinoceroses. Re- 
 mains of these animals are still uncovered frequently in Illinois. (Miniature scene 
 by Eric Lindgren in Museumobile. Photograph by Charles Hodge.) 
 
 ing these glacial periods to form their present channels; and many 
 of the small lakes that now dot the landscape in northern Illinois 
 and Wisconsin lie in depressions that were formed by glacial action. 
 
 What caused all this ice? What forces were active enough to 
 send not only one but four separate sheets of ice grinding southward 
 into Illinois? No one yet knows exactly what was the cause of these 
 repeated glacial climates, but most geologists believe that there must 
 have been several factors acting together to bring about the glaciers. 
 Some of the suggested causes have been elevation of the continents 
 or parts of the continents, changes in atmospheric conditions causing 
 colder climates over long time periods, much precipitation in the form 
 of snow and ice, tilting of the earth's axis causing the sun's rays to 
 strike the earth at a different angle resulting in colder climates, and 
 possible variations in the amount of the sun's heat. Some or all of 
 these factors may have acted at the same time to produce glaciers. 
 Some geologists think that only slight changes in temperature or in 
 the elevation of the land would be enough to send still another glacier 
 down from the North. We may now be living in an inter-glacial age, 
 and at some future time, perhaps thousands of years hence, the ice 
 will advance again over Illinois. 
 
 The animals which inhabited North America during the Ice Age 
 and its warm inter-glacial stages were fully as interesting as the spread 
 of the ice itself. There were elephants, huge mammoths and masto- 
 dons, giant beavers, camels, musk oxen, and ground sloths. Even now 
 
 — 24
 
 Contour plowing on a farm near DeKalb, Illinois, showing one way in which 
 man has learned to work with nature. These furrow hold rainfall which is 
 absorbed by the soil. (Photo by U. S. Soil Conservation Service.) 
 
 the remains of some of these animals are found in IlHnois and in other 
 states where the warm inter-glacial stages and the plentiful grasses 
 formed a favorable environment. Every few years or so, the huge 
 bones, tusks, or teeth of a woolly mammonth are found in Illinois, 
 mute evidence of the gigantic animals that roamed over our state. 
 
 So through all these millions of years, the geologic forces have 
 been at work, now building up and now tearing down, to sculpture 
 the land surface of Illinois as it appeared when first seen by man. 
 What man has done to and with Illinois is another story. However, 
 the geologic story does not end abruptly with the coming of man, for 
 the same forces are at work upon the face of the earth today, the same 
 forces of wind, water, earthquakes, and other earth movements, vol- 
 canoes, glaciers, and tides, the same forces that have been active for 
 millions of years. The better we understand the geologic forces, the 
 better we can work with them and not against them to maintain our 
 fertile soil, to conserve the natural raw materials formed millions 
 of years ago by geologic forces, and thus to insure an abundant life 
 for future generations. Unless we do these things, these same geologic 
 forces will work together for our certain destruction through our own 
 lack of vision. 
 
 If the geological story of Ilhnois makes the coming of man upon 
 the scene seem insignificant, it is only fitting because it took between 
 two and three billion years before man's arrival to fashion our present 
 landscape. Man's time on the earth, about a million years, has been 
 a fleeting moment compared to this vast period preceding his com- 
 ing. Yet the geologic story of Illinois promises some hope for the 
 
 — 25 —
 
 future of the earth, because the past earth history does show progress 
 among the chief plant and animal forms. In past geologic ages, 
 progress among plants and animals depended entirely upon uncon- 
 scious natural forces. But now we seem to stand at a turning point 
 in the development of life on this earth. It is clear that any future 
 progress or failure depends to a considerable extent upon man, the 
 first animal able to reason and to control, to a limited degree, natural 
 forces. As thinking, reasoning individuals the burden of the future 
 is upon us. Although we may be insignificant as individuals, col- 
 lectively as the entire human race we shall doubtless play a greater 
 part in shaping the future of the earth than any other plant or animal 
 has been privileged to do. By our actions we can condemn huge 
 forests to extinction; we can forever destroy the passenger pigeon or 
 the buffalo. We can in one generation destroy a soil layer that re- 
 quired thousands of years to form in nature. In fact, if we so desire, 
 we can apparently wipe out all existing life on the earth, or a Pood 
 part of it, with a few properly constructed bombs. But we have the 
 alternative of using the reasoning powers given us to work out a suc- 
 cessful plan of life whereby we can fit in with the natural forces in- 
 stead of trying to subject the forces of nature to our own selfish plans. 
 If we are only a small part of this greater scheme of things, it should 
 give us a feeling of humility but should also make us immensely 
 proud that we are here to assume our small part and that we, unique 
 among all other living things of this earth, have been able to see and 
 to understand as much of the past as we have and to glimpse and to 
 conjecture about as much of the future as we do. 
 
 APPENDIX I 
 
 Note 1 — Age of the Earth 
 
 Many different methods of attempting to determine the age of the earth 
 have been tried by man. Once it was thought that the age of the earth could be 
 judged by determining the amount of salt in the seas or by calculating the total 
 thickness of sedimentary rocks that had been piled up throughout the course 
 of time. 
 
 At the present time, the most accurate method of age determination is cen- 
 tered around the fact that uranium disintegrates or breaks down at a constant rate 
 of speed, turning into a kind of heavy lead. By determining carefully the ratio 
 of uranium to lead in certain rocks of the earth's crust, it has been possible to set 
 the age of the earth at some place between two and three billion years. This 
 "radioactive disintegration" method of age determination seems to be fairly fool- 
 proof because the rate of change from uranium to lead is not affected by heat, 
 pressure, or other external changes that may occur around the uranium-bearing 
 mineral. 
 
 Note 2 — Fossils and How They Are Formed 
 
 A fossil is defined simply as any evidence of pre-historic life. Thus fossils 
 really include such indications of life as footprints, casts of worm tracks, and 
 coprclites or fossil dung as well as the remains of the plant or animal itself. 
 
 In most cases, two conditions are necessary for the preservation of fossils. 
 One of these conditions is the possession of hard parts. It is infinitely more 
 possible for any animal with hard parts such as a shell or a backbone to 
 leave a fossil record than for some soft-bodied animal like a jellyfish or a worm. 
 The other condition that greatly aids in the formation of fossils is immediate 
 burial after death. This immediate burial of the plant or animal may come 
 about in a number of ways, such as in a tar or asphalt pit, in a body of water 
 where sediments cover the plant or animal, or under a fall of volcanic ash. The 
 
 — 26 —
 
 important thing is that immediate burial halts the decay that would normally 
 occur and also keeps the animal from being devoured by scavengers. 
 
 Note 3 — The Origin of Sedimentary Rocks 
 
 There are three great classes of rocks, including the igneous ("fire-formed"), 
 the metamorphic ("changed"), and the sedimentary (those formed from sedi- 
 ments). The sedimentary rocks contain most of our oil and coal, important ore 
 deposits of iron and other minerals, as well as most of our subsurface water 
 supply. These sedimentary rocks are of three principal kinds — sandstone, formed 
 by the cementing together of sand grains; shale, formed of hardened clay or 
 mud; and limestone, composed of hardened lime, animal shells, and other limy 
 remains. All sedimentary rocks are thus secondary rocks, formed from some 
 pre-existing rock or animal remains. The processes of the creation and destruc- 
 tion of sedimentary rocks are in progress continually. In some places, rocks are 
 being broken up by chemical and mechanical weathering only to be re-deposited 
 at some other place, where new sedimentary layers are being formed from the old. 
 
 APPENDIX II 
 
 Explanation of Terms Discussed in Text 
 
 Algae — one-celled plants that possess green coloring matter called chlorophyll. 
 The algae are thought to have been among the very early forms of plant 
 life and include the seaweeds. 
 
 Angiosperm — seed bearing plants which have the seed completely enclosed in 
 tissue, as in the pea or apple. These are the flowering plants, although 
 often the flower is not very showy or large. 
 
 Brachiopods — marine, invertebrate animals, possessing bi-valved shells. The two 
 shells or valves are unlike but each shell shows bilateral symmetry. 
 
 Bryozoans — marine and fresh-water animals that live in colonies on the sea floors. 
 Sometimes called sea moss. 
 
 Cephalopods — single-shelled marine invertebrate animals whose shell may be 
 straight or coiled, consisting of several separate chambers. Includes the 
 pearly nautilus. 
 
 Corals — invertebrate animals of warm shallow waters. Some live alone as single 
 individuals; others form large colonies or coral reefs. "Horn corals" or 
 solitary corals form a hard, calcareous secretion which is preserved in fossil 
 form. Colonial corals often resemble honeycomb structure. 
 
 Crinoids — marine invertebrate animals sometimes called "sea lilies". When 
 alive, the animal is generally attached to sea floor by a stalk and its mouth 
 is surrounded by long, feathery arms. 
 
 Eurypterids — segmented animals averaging 2-3 inches in length. All are now 
 extinct. Reached their climax in Silurian and Devonian Periods. They 
 were carnivorous and resemble trilobites in general body form. 
 
 Foraminitera — a large group of single-celled animals, mostly marine and gen- 
 erally microscopic in size. Body protected by a shell. Often found in oil- 
 well cuttings and are important in determining age of rocks in oil work. 
 
 Gastropods — single-shelled invertebrate animals, including many extinct forms as 
 well as numerous living forms like the slugs and snails. The shell, if coiled, 
 is always conical. 
 
 Graptolites — a group of entirely extinct animals that were very abundant in 
 the Ordovician and Silurian Periods of time. Graptolites were small, float- 
 ing animals which formed long, thread-like colonies. 
 
 Gymnosperms — seed bearing plants which have the seed growing on the sur- 
 face of the seed organ, not enclosed in the tissue of the parent plant. 
 Examine a pine cone to see how the seed is on the surface of the cone 
 scale. 
 
 Ostracoderms — a group of primitive fishes possessing a protective skin cover- 
 ing on the back composed of small, interlocking plates of chitin which are 
 preserved as tossils. Thought to be the first fish, appearing in the 
 Ordovician. 
 
 Trilobites — extinct marine animals having a three-lobed body form and resembling 
 modern crags or lobsters. Generally 1-3 inches long. 
 
 — 27 —
 
 The Kanic West Chicago Moraine in Kane County. In the fore-ground is a 
 glacial lake. Location: Township 42N, 8E, Section 1. (Courtesy of the Illinois 
 State Geological Survey.) 
 
 A wooded kame in McHenry County. Location: Township 43N, 8E, Sec- 
 tion 14. (Courtesy of the Illinois State Geological Survey.) 
 
 28
 
 A wooded esker in McHenry County. Location: Township 43N, 8E, Sec- 
 tion 14. (Courtesy of the Illinois State Geological Survey.) 
 
 A huge granite boulder found in the Kankakee, Illinois, quadrangle. This 
 was transported from its outcrop area in the north by the glacial ice sheets. Loca- 
 tion: Township 30N, 14 W, Section 12. (Courtesy of the Illinois State Geological 
 Survey. ) 
 
 29 —
 
 N 
 
 i 
 
 
 a JO ic JO 10 so 
 
 1 .1 I I 1. 1 1 ' I I 
 
 in 
 
 PLATE I 
 SKETCH MAP 
 
 v^vinp {he DisVributioo of 
 
 BEDROCK STRATA 
 
 Illir7oi&' 
 
 -In man\f places 4bc-s<t 
 rocks are. cover ad by 
 plc<*i-oc«rj«. deposits 
 
 Sea- T>lah£ IT 
 
 Explanation of the Map 
 ID Tertiary ■Devonian 
 
 ffiiCretaceous D Silurian 
 
 ^Pennsylvanian E3 0rdovician 
 OMississippian E^Cambrian 
 
 — 30 —
 
 N 
 
 A 
 
 PLATE 
 
 SKETCH 
 
 S^>ov^/Ing 4hc Bisiribu-fion o-f 
 
 PLEISTOCENE DEPOSITS 
 
 in Illinois 
 
 Explanation of the Map 
 
 □ Wisconsin 
 @ lllinoian 
 
 CZZI Kansan 
 
 I I Unglaciated 
 
 (possibljy some Nebraskan ) 
 
 — 31 
 
 •■ » #'A*'A'AA 
 
 - ' rA''A'A\-' 
 
 'Ti
 
 GEOLOGIC TIME UNITS 
 
 DISTINCTIVE FEATURES OF EACH 
 PERIOD 
 
 Dominant 
 Animals 
 
 Dominant 
 Plants 
 
 
 Quarternary 
 Period 
 
 Ri'cont Epoch 
 
 Today geologic [jrocesses continue in activ- 
 ity as in the past. New volcano, Parlcutin, 
 is formed in Mexico. Glaciers melt slowly 
 in U. S. and Canada. 
 
 Age of Mammals 
 
 i2 
 o 
 
 o 
 
 o 
 
 UJ 
 
 
 Pleistocene 
 Epoch 
 
 Stone Age in human history. Widespread 
 glaciation with alternating warmer periods. 
 Mountains attain present heights. 
 
 Cenozoic Era 
 
 T3 
 O 
 
 >> 
 Eh 
 
 Pliocene Epoch 
 
 Climate becomes cooler and drier. 
 First appearance of man. 
 
 w 
 
 Miocene Epoch 
 
 Mammals reach climax. Grazing types 
 evolve as prairies spread. Elephants reach 
 America. 
 
 t3 S 
 
 
 Oligocene 
 Epoch 
 
 Mammals evolve rapidly. Great apes ap- 
 pear in Eurasia. 
 
 p-i 
 
 
 Eocene Epoch 
 
 Modern orders of mammals appear and 
 evolve rapidly. 
 
 a 
 
 eo.nnn.noo 
 
 yours ago 
 
 Paleocene 
 Epoch 
 
 Archaic mammals dominate. Small valley 
 glaciers appear locally. 
 
 o 
 
 S 
 
 Mesozoic Era 
 
 Cretaceous Period 
 
 Dinosaurs, pterodactyls, toothed birds reach 
 climax, then die out rapidly. Small mam- 
 mals appear. Flowering plants and hard- 
 wood forests spread widely over continents. 
 
 p. 
 
 « 
 
 "o 
 
 < 
 
 o 
 
 Jurassic Period 
 
 Dinosaurs and marine reptiles dominate. 
 Toothed birds appear. Ammonites reach 
 climax of development. 
 
 Age of 
 Conifers 
 
 2UU,00U,0(1U 
 years ago 
 
 Triassic Period 
 
 Small dinosaurs and first mammals appear. 
 Conifers and cycads dominate forest. Am- 
 monites develop rapidly. 
 
 
 Permian Period 
 
 Uplift of continents widespread. Extremes 
 of heat and cold result in rapid evolution 
 and in many extinctions among plants and 
 animals. 
 
 ■a 
 
 
 bJ) 
 
 
 Paleozoic Era 
 
 Pennsylvanian 
 Period 
 
 3 
 
 e 
 
 'S 
 
 o 
 
 5 
 O 
 
 Mild climate favors plant growth and coal 
 formation. Reptiles and insects appear. 
 Seed ferns dominate swamp forests. 
 
 9'2 
 
 
 Mississippian 
 Period 
 
 Shell-crushing sharks, crinoids, and bryo- 
 zoans reach climax of development. 
 
 a 
 
 
 Devonian Period 
 
 Lung-fishes evolve into air-breathing verte- 
 brates (amphibians). First forests appear. 
 Brachiopods reach climax. 
 
 
 Silurian Period 
 
 Climates warm. Corals form widespread 
 reefs. Very first evidence of land life found 
 here. 
 
 a* 
 
 g 
 
 > 
 p 
 
 
 
 Ordovician Period 
 
 Seas spread widely over low continents. 
 Many groups of invertebrate animals make 
 first appearance here. 
 
 
 550,000,000 
 years ago 
 
 Cambrian Period 
 
 Abundant fossils for first time. Marine life 
 only. Trilobites and brachiopods dominate. 
 Only plants are algae. 
 
 "s 
 
 Proterozoic 
 Era 
 
 Keweenawan Period 
 
 Vast lava flows and thick red sediments 
 cover Lake Superior region. 
 
 bD 
 
 5 
 
 1,550,000,000 
 years ago 
 
 Huronian Period 
 
 ■Widespread glaciation in Canada. Lake 
 Superior iron ores are deposited. 
 
 
 
 Algoman Period 
 
 Great earth movements with high moun- 
 tains formed, intrusion of much granite, 
 then long erosion. 
 
 
 Archeozoic 
 Era 
 
 Timiskaming Period 
 
 Long period of accumulation of sediments 
 followed by profound erosion of the "Ca- 
 nadian Sliield" area. 
 
 ■< 
 
 
 Laurentian Period 
 
 Great earth movements and intrusions of 
 granites, followed by long period of erosion. 
 
 
 2,550,000,000 
 years ago 
 
 Keewatin Period 
 
 Thick lava flows in Lake Superior area in- 
 terfiiiger with oldest known sedimentary 
 rock of the world. 

 
 w^mmm^^'m:'^'::^- .. 
 
 '' J V ' 9 * It! 1 k C 1 1 ^ I ICI 
 
 Exploring 
 
 Flow< I 
 
 .' Invitation to Birds, by V. S, fei 
 , Man's Vpntiire ^n Culn?r 
 
 ^r Common Insects oi lu.noisv, ./> . 
 
 Wrig! 
 
 Address all enq ■ , 
 
 MUSEUM DIRECTOR, ILLINOIS STATE MUSfetTM, 
 
 Springfield, Hi 
 
 n . ^^.'aT/.
 
 UNIVERSITY OF ILLINOIS-URBANA 
 507IL61ST C006 
 
 STORY OF ILLINOIS SERIES. SPRINGFIELD 
 7 1951