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Digitized by the Internet Archive 
 
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 University of Illinois Urbana-Champaign 
 
 http://archive.org/details/storyofgeologicm01well 
 
Tower Rock, Hardin County 
 
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STATE OF ILLINOIS 
 DEPARTMENT OF REGISTRATION AND EDUCATION 
 
 M. F. Walsh, Director 
 
 DIVISION OF THE 
 
 STATE GEOLOGICAL SURVEY 
 
 M. M. LEIGHTON, Chief 
 
 EDUCATIONAL SERIES NO. 1 
 
 The Story of the Geologic Making 
 of Southern Illinois 
 
 BY 
 
 STUART WELLER 
 
 Printed by authority of the State of Illinois 
 
 URBAN A, ILLINOIS 
 
 1927 
 
 Reprinted 1031 
 
Department of Registration and Education 
 
 State Board of Natural Resources and Conservation 
 
 Committee on Geological Survey 
 
 M. F. Walsh, Chairman 
 
 Director of Registration and Education 
 
 Charles M. Thompson 
 
 Representing the President of the Uni- 
 versity of Illinois 
 
 Edson S. Bastin 
 
 Geologist ? 
 
 Jeffersons Printing & Stationery Co. 
 
 Springfield, Illinois 
 
 1931 
 
Letter of Transmittal 
 
 State Geological Survey Division, Urbana, May 27, 1931 
 
 M. F. Walsh, Chairman , and Me tubers of the Board of 
 Natural Resources and Conservation , 
 
 Gentlemen: 
 
 The first edition of Education Series No. 1, "The Story 
 of the Geologic Making of Southern Illinois," by the late 
 Professor Stuart Weller, has become exhausted and a second 
 edition is necessary to meet the demands of our public schools 
 and of our laymen for this popular treatment. I therefore 
 respectfully recommend its reprinting. 
 
 Sincerely yours, 
 
 M. M. Leighton, Chief. 
 

Preface 
 
 In southern Illinois lies a region which occupies about one 
 twenty-eighth of the area of the State and which differs marked- 
 ly in appearance from the rest of the State. By far the greatest 
 portion of Illinois is made up of flat, fertile prairies; indeed, 
 in the central section, hills are exceedingly scarce. A traveler 
 making his way south from Harrisburg in Saline County finds 
 that he has left the prairies behind and that he is among hills, 
 the highest one of which — Williams Hill in Pope County — 
 attains an elevation of 1,065 feet. In no other part of the 
 
 CAVE IN-ROCK 
 ROSICLARE 
 
 |5 MILCS 
 
 Fig. 1. — Map of southern Illinois showing approximate boundaries 
 of Ozark Ridsre. 
 
 State save in Jo Daviess County in the northwest corner are 
 there higher hills. The traveler finds short, swift streams and 
 steep ascents, hidden nooks and outlook points, and a relief 
 made most engaging to the eye In reason of rapidl) changing 
 
scenes. And nowhere does he find the extensive fields and 
 farms so characteristic of the prairies. 
 
 This area of southern Illinois covers a strip about 70 miles 
 long and from 15 to 40 miles wide (fig. 1). It extends from 
 Mississippi River on the west to Ohio River on the east, and 
 lowlands lie both to the north and to the south of it. 
 
 The history which follows attempts to outline briefly the 
 manner in which southern Illinois came to be as we know it. 
 Much detail necessarily has been omitted, and much remains to 
 be learned, but not a year passes without the acquisition of 
 some new information which helps to complete the story. 
 
THE STORY OF THE GEOLOGIC 
 MAKING OF SOUTHERN ILLINOIS* 
 
 By Stuart Welter 
 INTRODUCTION 
 
 When man began to wonder about the reasons for things 
 and began to seek out the causes for the great variety of natural 
 phenomena which surrounded him, he began the ascent of the 
 intellectual ladder many of whose steps he has already climbed, 
 but from the top of which he is still much farther removed than 
 from the bottom. In the early stages of this intellectual growth 
 he had no perception of natural law and he of necessity had to 
 explain the phenomena of nature through supernatural agencies. 
 Thus grew up the myths of creation as well as other myths and 
 stories to account for all of the unexplainable phenomena about 
 him. Many, many generations passed before man's intelligence 
 began to give him some conception of the workings of the laws 
 of nature. Undoubtedly, from time to time some great genius 
 arose whose sense of curiosity was more intense than that of 
 his fellows, — one who was capable of searching deeply into the 
 reasons for things and who was able to make some addition to 
 the knowledge of the human race. Comparatively few genera- 
 tions of men have passed since Newton formulated the law of 
 gravitation, perhaps the most far-reaching of all natural laws. 
 Copernicus, who founded all modern astronomy by his dis- 
 covery that the earth was not stationary but revolved about 
 the sun, was born less than 500 years ago. Even today the 
 vast majority of human beings, and among them great numbers 
 of our own people, have but little conception of the meaning of 
 
 * This paper is based on an address given by the author as retir- 
 ing- president of the Illinois State Academy of Science at the Harris- 
 burg meetings, April 30, 1926, and published in the Proceedings of the 
 Academy, vol. 19, pp. 27-49. 
 
FEATURES OK THE LANDSCAPE 
 
 science, and still hold to all sorts of supernatural explanations 
 of phenomena which they observe every day. 
 
 Reduced to its last analysis, science is the search for the 
 reasons for things. Every man of science knows that every 
 phenomenon in nature has a cause, a reason, and just as truly 
 every phenomenon has an effect. The search for the causal 
 relations of all these phenomena is his task. If he is unable 
 to discover a reason for the facts which he observes, he does 
 not appeal to some supernatural explanation ; he knows that his 
 researches have not yet penetrated far enough into his problem. 
 The men of science of our generation are those in whom the 
 sense of curiosity concerning nature is most keenly developed. 
 They are continually asking themselves the question, "Why?" 
 and are never satisfied until some reasonable and logical ex- 
 planation is found. Every child starts his career as an active 
 researcher. He observes all sorts of phenomena about him 
 which arouse his curiosity, and he repeatedly asks the question 
 "Why?" All too often his questioning soon surpasses the ability 
 of his parents and teachers to make answer and he is told not 
 to ask such foolish questions. Doubtless many an able man 
 of science has been lost through the stifling of childish question- 
 ing. Fortunately a few remain in whom this curiosity cannot 
 be stifled, else the progress of our race would come to a stand- 
 still. 
 
 EARLY CONCEPTIONS 
 
 Among the phenomena which early attracted the attention 
 
 of man, were the features of the landscape which surrounded 
 
 him, the hills and valleys, the mountains and the sea. Because 
 
 „ r of their apparent stability, explanations for these 
 
 Features of , . 
 
 the phenomena were not sought as early as were 
 
 landscape explanations for the apparent movement of 
 
 heavenly bodies. If any thought was given to the sub- 
 ject, it was assumed that the hills and valleys and moun- 
 
GEOJ OGIC TIME AND PROCESSES I I 
 
 tains and seas had always been and would always be. This 
 conception of permanence is still with us and not infrequently 
 we see references to the everlasting hills, although every geolo- 
 gist knows that neither the hills nor the highest mountains are 
 everlasting. 
 
 Another group of objects which came to the attention of 
 early observers were the fossil shells seen in all sorts of situ- 
 tions. During the Middle Ages fossils aroused much discussion 
 and numerous explanations were offered for 
 them. Some thought them to be sports of 
 nature; some believed them to be discards thrown aside by 
 the Creator of all things ; but the final explanation which 
 seemed to satisfy most men was that they were the remains 
 of sea animals which were swept from the sea and were lodged 
 upon the land during the Noachian Deluge. Of course the 
 geologist today knows that these objects are the remains of 
 animals and plants, many of them marine, which one time 
 lived and flourished in the places in which the evidences of 
 their existence are now found. At that time the ocean waters 
 covered those areas now dry land but formerly parts of the 
 sea bottom. 
 
 GEOLOGIC TIME AND PROCESSES 
 
 It was only a little more than a hundred years ago that 
 students of geology began to realize that the present configura- 
 tion of the earth's surface has been established through the 
 agency of slowly acting processes which are in operation today 
 and which have been continuously in operation for long periods 
 of time, rather than through abrupt, cataclysmic revolutions 
 which occurred at comparatively short intervals. We have come 
 to realize that geologic time is long — exceedingly long when 
 measured by the scale of human existence — and we look upon 
 its duration as being comparable, in a degree, to the vast 
 
12 GEOLOGIC TIME AND PROCESSES 
 
 extensions of space recognized by the astronomer. And with an 
 increase in our knowledge of the earth's history comes an ex- 
 pansion in our conception of geologic time. In the history of 
 the earth a million years are but as a day, and forces and 
 processes which seem to have only a small effect when seen from 
 day to day are capable of producing tremendous 
 
 Modern effects when continued over long periods of 
 
 conceptions . . ° 
 
 time. During a summer shower, all of us have 
 
 seen material transferred from the hilltops to the valleys and 
 if enough time is allowed and enough showers fall upon a hill, 
 eventually that hill will be leveled to the plains. In the geo- 
 logic past this very transfer of material has taken place repeat- 
 edly. Mountains as high as the highest upon earth today have 
 been eroded down to the level of the sea; other mountains 
 have been raised up and have in turn been worn away. 
 
 The geologist interprets the history of the earth's past re- 
 corded in the rock strata of its crust through an understanding 
 of the processes which are active today. It goes without saying 
 that in the erection of a pile of lumber the boards in the bottom 
 of the pile were laid down first. Likewise the lowermost beds 
 in the rock strata of the earth's crust in any given region were, 
 axiomatically, laid down first and consequently are the oldest. 
 Figure 2 shows horizontal beds of limestone. In order for the 
 lowermost bed to have its position with respect to the other 
 beds above it, it had to be laid down first. Hence, it is the old- 
 est bed, and the beds above it are successively younger. 
 
 If the solid rock formations were everywhere uncovered 
 and had never been disturbed, it would be a comparatively 
 simple matter to trace the beds from place to place and estab- 
 lish their true relations. In most regions, how- 
 Correlation ever> these conditions do not exist. The rocks 
 are covered in many places by unconsolidated 
 residuum and soil. In parts of Illinois whole counties are 
 
CORRELATION OF ROCKS 
 
 13 
 
 without a single solid rock outcrop; in many others the out- 
 crops are so few and so widely separated that the beds cannot 
 be traced. Then again, through the agency of great deforma- 
 tive processes the rocks have been broken into blocks which 
 have been shifted up and down and horizontally to such an 
 extent that the individual beds are no longer continuous. 
 
 Under these conditions it has been necessary for the geol- 
 ogist to establish some means of tracing rock beds from place 
 
 Fig. 2. — Exposure of limestone beds in Kankakee County, Illinois. 
 
 to place, and long study has shown that the only reliable 
 criteria are the organic fossils contained in the rocks. 
 These fossils furnish a record of the evolution of the life 
 of our globe, the scattered remains of succeeding generations 
 which exhibit the gradual changes through which living crea- 
 tures have been passing. The life of no period of geologic time 
 is the same as that of the preceding or of the succeeding period ; 
 
14 GEOLOGIC TIME AND PROCESSES 
 
 consequently a knowledge of the succession of living things upon 
 the earth affords us a most reliable criterion for the determina- 
 tion of the relative ages of the rock strata which come under 
 our observation. It must be recognized that our knowledge of 
 these long-extinct inhabitants of the earth is as yet but frag- 
 mentary, but every year adds to our information, and already 
 we can make a serviceable application of accumulated facts in 
 our interpretation of geologic history. With further informa- 
 tion greater and greater refinements in our interpretations will 
 become possible. 
 
 If the earth's crust had undergone no deformative changes 
 during its long period of existence, all sediments which were 
 accumulated under marine waters and in which are buried the 
 
 ., remains of marine organisms, would still be sub- 
 
 Movements . . . . ^ 111- Ml 
 
 in earth's marine in position and would be inaccessible for 
 
 crust study. It is known, however, that crustal move- 
 
 ments have taken place even in historical time. Certain 
 portions of our shorelines are known to be rising relative 
 to the ocean level and other portions are sinking. Not in- 
 frequently violent earthquakes occur in the course of which 
 notable crustal changes take place. Rocks which are mani- 
 festly of marine origin since they contain fossil shells of marine 
 organisms, are now found thousands of feet above sea level; 
 they must have been elevated to their present position or the sea 
 must have receded. Probably both elevation and recession 
 occurred. Rocks which must have been formed originally as 
 nearly flat-lying sediments deposited in water, are now steeply 
 inclined in many places, and are even folded, crushed and 
 broken. The existence of tremendous forces which have been 
 at work in earth-growing processes is thus made manifest. 
 
 No sooner was land exposed above the level of the sea than 
 it was subjected to the processes of erosion. Water falling as 
 rain and always seeking a lower level started to wear away the 
 
DIVISION OF GEOLOGIC TIME 15 
 
 ^ . elevated rocks and gradually wore them down to 
 
 Erosion . . 
 
 of earth's sea level, just as the hills between Harrisburg 
 
 surface an( j Ohio River are at present being gradually 
 
 reduced by every rain which falls upon them. 
 
 Deformation and unequal erosion have resulted in the 
 exposure somewhere at the surface of the earth of sedimentary 
 rocks of all ages. In future geologic periods, rocks which now 
 exist at the surface will have been worn away and their mate- 
 rials will have been carried down the streams to the ocean and 
 redeposited to form younger rocks, much as sediments are now 
 accumulating in the delta of Mississippi River. 
 
 Our present knowledge of the earth has progressed far 
 enough to make it possible for geologists to recognize a number 
 of distinct divisions of geologic time. Of course time is con- 
 tinuous and uninterrupted, and we can divide it 
 
 Division of - nt0 d^tinct periods only on the basis of events, 
 geologic time ..... . . 
 
 Our subdivision of time in human history has 
 
 been wholly dependent upon the occurrence of events. The 
 discovery of a new continent in the fifteenth century ini- 
 tiated into human history a new period of the utmost 
 importance. The events which led to the declaration of 
 war in Europe in 1914 ushered in a period which has changed 
 and is changing the destinies of most of the nations of the world. 
 The transition of one period into another may be gradual like 
 the passing of spring and the coming of summer, or it may be 
 abrupt and unexpected like the beginning of the great World 
 War. The periods of geologic history are not unlike those of 
 human history. They are definitely marked from one another 
 by events like the growth of great mountain ranges, or the 
 appearance of a new type of life. Some transitions have been 
 abrupt and have been accomplished in an exceedingly short 
 time, geologically speaking; others have been so slow as to 
 
16 GEOLOGIC TIME AND PROCESSES 
 
 make difficult the setting of a precise boundary between two 
 rock systems although each has distinct characteristics. 
 
 It has come to be the custom of geologists to use as their 
 
 basis for the larger divisions of earth history the character of 
 
 the life of the era. Azoic time was an era during which no 
 
 life existed. Eozoic was the era of the dawn of 
 
 Eras of ^ life with the introduction of unicellular organ- 
 geologic time . r 
 isms. 1 he Archeozoic era was the age 01 
 
 larval life, and the Proterozoic the age of primitive inverte- 
 brates. All earth history to the close of Proterozoic time 
 is so ancient that our information regarding it is most hazy. 
 The Proterozoic was followed by eras concerning which our 
 information is less limited although it is far from complete. 
 The Paleozoic era was the time of ancient life forms, now 
 mostly extinct, but some of them were more or less related to 
 our contemporaneous life. The Mesozoic era was the time of 
 medieval life which, although almost wholly unlike living forms 
 of today, still had much more in common with them than had 
 the life of the Paleozoic. The Cenozoic era was the time of 
 modern life which was much like that of today. The last, or 
 Psychozoic, era is the age of man. 
 
 Each one of these great eras is divided into periods some 
 
 of which have been named geographically after some district 
 
 in which rocks representing the period are well exhibited. One 
 
 great period of the Paleozoic era has been 
 
 Periods of named the Devonian because it was first studied 
 geologic time 
 
 and its fossil life was first described at Devon- 
 shire, England. Following the Devonian w r as a great period 
 known as the Mississippian, so named because its rocks are 
 best represented in the Mississippi Valley region of this coun- 
 try. The Mississippian was succeeded by a period during 
 which the great coal formations of eastern North America were 
 
DIVISION OF GEOLOGIC TIME 
 
 17 
 
 laid down. The name Pennsylvanian has been given it because 
 the rocks are widely developed in Pennsylvania. 
 
 Table 1. — Table of geologic time divisions. 
 
 Era 
 
 Period (time term) or 
 System (rock term) 
 
 Character of rocks in 
 Southern Illinois 
 
 Cenozoic 
 
 "Recent life" 
 
 (Age of mammals) 
 
 Recent 
 
 Pleistocene 
 
 Tertiary 
 
 Surficial deposits 
 Absent 
 
 Mesozoic 
 
 "Middle life" 
 
 (Age of reptiles) 
 
 Cretaceous 
 Comanchean 
 Jurassic 
 Triassic 
 
 Absent 
 
 Paleozoic 
 "Ancient life" 
 (Era of invertebrate 
 animals and non- 
 flowering plants) 
 
 Permian 
 Pennsylvanian 
 (Coal Measures) 
 Upper 
 Mississippian 
 
 Lower 
 Devonian 
 Silurian 
 Ordovician 
 Cambrian 
 
 Absent 
 Conglomerates, sandstones, 
 
 shales, and coal 
 Sandstones, shales, and 
 
 limestones 
 Principally limestones 
 Limestone and shale 
 Limestone 
 
 Limestone and sandstone 
 Not exposed 
 
 Proterozoic 
 "Primitive life" 
 
 
 
 Archeozoic 
 "Larval life" 
 
 
 
 Table 1 gives the eras and periods of geologic time. As the 
 Archeozoic era is the oldest that we know and its rocks 
 were formed first, it is placed at the bottom of the list. 
 Similarly, the Cenozoic era is the youngest and is placed at 
 the top of the column. More detailed information regarding 
 the geologic time table and geologic principles can be found in 
 the standard text books on geology. 
 
18 
 
 ROCKS OF SOUTHERN ILLINOIS 
 
 ROCKS OF SOUTHERN ILLINOIS 
 
 By reason of the processes which have been described all 
 too briefly, rocks belonging to the Ordovician, Silurian, Devon- 
 ian, Mississippian, and Pennsylvanian periods are now exposed 
 at the surface in southern Illinois. 
 
 The Ordovician and Silurian rocks are exposed in the 
 Mississippi River bluffs in western Alexander County. Devon- 
 ian rocks are found in two areas; (1) along Mississippi 
 River in Alexander County, Union County, and a very small 
 part of Jackson County; and (2) in Hardin County. 
 
 Fig. 3. — Diagram to illustrate effect on horizontal beds of intru- 
 sion of molten rock or lava (A). 
 
 The second area of Devonian outcrop is limited and is 
 known to geologists as Hicks Dome. It is a structure which is 
 comparable to a doming of the earth's crust. Figure 3 illustrates 
 the probable manner of formation of this dome. 
 The rock strata were at one time nearly hori- 
 zontal. A mass of material (A in the diagram), probably in 
 
 Hicks Dome 
 
DEVONIAN AND OLDER ROCKS 19 
 
 the form of molten rock or lava, was injected into the crust 
 beneath the horizontal strata. This mass pushed up the beds 
 above it and a doming at the surface resulted. The molten 
 material is now completely cooled and solidified. After the up- 
 lifting, erosion began on the higher parts of the dome and 
 the overlying strata were worn away until now the lower and 
 older beds are exposed in the center of the dome. Figure 4 
 is a geologic cross-section of Hicks Dome, in which the vertical 
 scale has been much exaggerated. 
 
 Wherever a deep drill-hole is sunk for oil or for other 
 purposes, the geologist can make logical deductions from the 
 information which it gives regarding the rocks penetrated. 
 
 ~ . So we know that the Devonian rocks continue 
 
 Devonian 
 
 and older beneath the surface from the area of exposure in 
 
 rocks Hardin County, across southern Illinois into 
 
 Union County where they are again exposed, and into south- 
 Hicks Dome 
 
 Fig. 4. — Geologic cross-section of Hicks Dome, Hardin County. 
 
 eastern Missouri. To the east and south they are com- 
 pletely hidden by younger, overlying strata. Of course the 
 Devonian rocks are not the oldest rocks in southern Illinois. 
 We know about the rocks and their attitudes in other 
 parts of the Mississippi and Ohio valleys and, making logical 
 deductions, we know that beneath the Devonian rocks of 
 Hardin County are formations representative of all the major 
 geological systems below the Devonian. Some of these older 
 rocks, as stated above, are exposed in the Mississippi River 
 bluffs. 
 
20 
 
 ROCKS OF SOUTHERN ILLINOIS 
 
 The southern slope of the Ozark hills south of Harris- 
 burg is underlain by rocks of Mississippian age; the higher 
 parts are composed of Pennsylvanian strata. These Pennsyl- 
 
 Mississippian Vanfan r0cks dip t0 the north ' and at Harris " 
 and Pennsyl- burg the strata which are exposed at the tops 
 vanian rocks of the Qzark hiUs are found faf beneath the 
 
 surface. Figure 5 shows this condition in diagrammatic form. 
 The coal beds which are so extensively mined in southern 
 
 Norfli 
 
 South 
 
 Fig. 5. — Diagram illustrating dip of rocks south of Harrisburg. 
 Vertical scale much exaggerated. Bed A outcrops at the 
 surface at the top of the hill south of Rudement, but at Har- 
 risburg it is far beneath the surface. 
 
 Illinois belong stratigraphically far above the sandstone 
 which forms the tops of the Ozark hills. When they were 
 originally deposited, the coal beds were undoubtedly continu- 
 ous with the existing coal fields of western Kentucky, but sub- 
 sequently an area in Hardin County and adjacent portions of 
 Kentucky was uplifted and not less that 3,000 feet of hard rock 
 strata have been removed by the slow processes of erosion. The 
 Ohio River has cut its valley through all these strata. 
 
DEVONIAN SEA 21 
 
 HISTORY OF THE MAKING OF 
 SOUTHERN ILLINOIS 
 
 No history of the making of southern Illinois can be com- 
 plete without taking into account those rocks older than the 
 Devonian, but as the major events which led to the present 
 configuration occurred after this period, and as in only a small 
 area are older rocks exposed, we shall begin our story with 
 Devonian conditions. 
 
 The oldest Devonian rocks are limestones in which are 
 buried numerous fossil shells of marine animals, and we can 
 confidently assert that during Devonian times southern Illinois 
 was covered by a great shallow sea which ex- 
 tended north to Canada, northeast to New York 
 State, and east to the Allegheny Mountains. This sea had 
 great islands ; one extended north from where Cincinnati, 
 Ohio, now stands; another was in central Tennessee about 
 where Nashville now exists. We know from the fossil evi- 
 dence that this sea did not extend west of the Mississippi River 
 except in southern Missouri. To the south it must have had 
 connection with the open ocean. 
 
 At the end of Middle Devonian time, it is known that 
 
 great crustal disturbances took place in southeastern Missouri. 
 
 Great faults were formed with a maximum dislocation of the 
 
 Crustal beds of 1000 feet. There is no evidence to 
 
 disturbances i , . .1 , £ . ,. . , 
 
 in Devonian snow that any similar detormative disturbance 
 
 times took place in southeastern Illinois, but the great 
 
 earthquake tremors which the Missouri disturbances must 
 
 have created certainly affected this portion of Illinois. In 
 
 southeastern Missouri the earth movements left the ancient 
 
 sea bottom well above sea level. It is not improbable that 
 
 southern Illinois was also dry land for a time, although 
 
 it was certainly again submerged in late Devonian time when 
 
22 GEOLOGIC MAKING OF SOUTHERN ILLINOIS 
 
 it became a part of a widespread interior sea which extended 
 from the Appalachian Mountains on the east to Oklahoma on 
 the west, and as far north as Lake Erie at least. It extended as 
 far southeast as Chattanooga, Tennessee, but its southern ex- 
 tent is not known definitely, for the sediments deposited are 
 hidden under much younger rocks. 
 
 The character of the late Devonian deposits indicates that 
 the waters of this interior sea were more or less stagnant. The 
 sediments were fine muds with a large admixture of organic 
 
 _, c matter which gave the consequent shales their 
 
 Character of . B /*. 
 
 deposits in intensely black color, vjuantities or the minute 
 Devonian sea S p 0re cases of an ancient type of plant were 
 buried in these muds and are now preserved in the fossil con- 
 dition. Animal fossils are scarce, and those which are present 
 show that the sea was not a typical body of marine water and 
 that it had only indirect communication with the ocean water 
 surrounding the continent. 
 
 This great interior sea with its more or less stagnant 
 waters must have existed a long time, humanly speaking, for 
 no less than 400 feet of the characteristic black shales accumu- 
 lated, at least locally. The period, however, was brief as geologic 
 time is reckoned, and at its close the whole of southern Illinois 
 was elevated above sea level. Once again began the wearing 
 down processes of erosion which are always effective when a 
 land surface is elevated and whose vigor depends on the climate, 
 the elevation above sea level, and the distance from the sea. 
 
 At last the region was submerged again as a result of 
 crustal deformation or of a rise in sea level due to changes else- 
 where, and earth history passed from the Devonian into the 
 Mississippian Mississippian period. The Mississippian sea 
 sea which covered southern Illinois during this time 
 
 was a widespread body of water over the interior of the con- 
 tinent. It spread northward in the present Mississippi Valley 
 
MISSISSIPPIAN SEA 23 
 
 at least as far as Iowa and northern Illinois and doubtless at 
 times it extended into southern Wisconsin. At the time of its 
 greatest extent it spread as far to the northwest as Montana 
 and as far to the southwest as New Mexico. To the south- 
 east it reached Alabama and Georgia but on the east it was 
 limited by a great land mass known as Appalachia which 
 occupied a position east of the present Appalachian Mountains. 
 The Mississippian sea, no doubt, had free communication with 
 the open ocean to the south. 
 
 Conditions in the Mississippian sea were vastly different, 
 however, from those which gave rise to the great accumulation 
 of Devonian shales, for in it were deposited mainly limestones 
 whose fossil contents consist of typical marine organisms. 
 The oldest formations deposited in this Mississippian sea are 
 the limestone formations which encircle Hicks Dome. Younger 
 rocks are exhibited in the limestone bluffs of Ohio River which 
 are more or less continuous from Rosiclare to Cave-in-Rock, 
 in Hardin County. The same limestone formations are found 
 in that part of the county where sink holes are so numerous. 
 
 There are several distinct limestone formations in the 
 
 Lower Mississippian. These have been named the Iowan series 
 
 because they are well developed and were first described in Iowa. 
 
 They are distinguished by the character of the 
 
 Mississippian s tra t*i themselves and bv the differences in fossil 
 limestones 
 
 content. 1 he changes reflected by the forma- 
 tions were caused by changes in the boundaries of the sea, 
 in the depths of the water, and in the purity of the water. 
 The purer limestones consist largely of calcium carbonate 
 which was withdrawn from solution in the sea waters through 
 the agency of shell-forming animals. Some of the shells were 
 broken and even ground into fine powder by the move- 
 ments of the water and formed the lime muds of the sea 
 
24 GEOLOGIC MAKING OF SOUTHERN ILLINOIS 
 
 bottom; others were buried with little or no injury in these 
 lime muds. Doubtless many bacteria were present which caused 
 the precipitation of lime. 
 
 During part of the time, the Ozark region of Missouri 
 
 constituted a great island, Ozarkia, which furnished materials 
 
 to the sea to form shales, sandstones, and impure limestones. 
 
 Then again the island was entirely submerged 
 
 and, since the source of land-derived sediments 
 
 was removed purer limestones accumulated. 
 
 Cincinnatia was another great, low-lying island, which 
 extended north from Cincinnati, Ohio. East of it was an arm 
 of the sea, which extended into Michigan and in which little 
 
 limestone formed, since a continuous supply of 
 
 Cincinnatia i . i . , , , A 
 
 non-calcareous material was turnisned by Appa- 
 
 lachia. The shorelines of this great interior sea shifted con- 
 tinually by reason of the crustal movements of the earth, but 
 there is no evidence that southern Illinois was raised above sea 
 level before the close of the period. 
 
 The Iowan sea teemed with a great variety of life as to 
 
 form, but instead of the clams and snails which make up so 
 
 great a part of the fauna of our present oceanic waters, there 
 
 were great numbers of brachiopods and crinoids, 
 
 Life in the types of life which are met with onlv rarely in 
 Iowan sea J ' 
 
 the seas today. There were also a few trilo- 
 
 bites, a life form which is now wholly extinct, some snails, 
 clams, and corals. (See fig. 6.) All these forms were 
 creatures which formed hard, stony shells which were cap- 
 able of fossilization. There were great numbers of fishes also, 
 all of them related to the sharks. Some must have grown to a 
 large size; their sole records are their fossilized teeth and fin 
 spines. There were doubtless also many soft-bodied animals, as 
 there are in the seas today, which were wholly incapable of 
 leaving records of their existence, as well as sea weeds of va- 
 rious sorts, 
 
LIFE IN THE IOWAN SEA 
 
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26 GEOLOGIC MAKING OF SOUTHERN ILLINOIS 
 
 Following the long period of limestone formation, southern 
 Illinois was raised above sea level; indeed it is not unlikely that 
 the whole of the interior of the continent became dry land. 
 Processes of erosion became active but the low relief of the 
 land prevented rapid erosion and no great thickness of the 
 limestone was removed. The emergent condition continued 
 for a considerable length of time but not so long that some 
 forms of animal life which had inhabited the Iowan sea sur- 
 vived elsewhere and re-entered the area with a new period of 
 submergence. 
 
 The new period of submergence in southern Illinois 
 
 brought with it a new set of conditions. The rock-forming 
 
 materials were no longer almost pure lime, but included also 
 
 great quantities of sand and fine mud which 
 
 Chester built up the Chester series. At the present 
 
 embayment ^ . . [ 
 
 time the Chester formations constitute the sur- 
 face rocks upon much of the southern slope of the Ozark hills 
 in southern Illinois and in places they continue to the Ohio 
 River bluffs and even across the river into Kentucky. The basin 
 in which this series of rocks accumulated was very different 
 from the great interior sea in which the limestones of the Iowan 
 series were formed. In Illinois the basin w T as simply a great 
 embayment which extended northward between Cincinnatia on 
 the east and Ozarkia on the west, to about the middle of the 
 State. Into this embayment emptied great rivers whose sources 
 doubtless were far to the north in what we now know as Can- 
 ada. These rivers brought down much sand and fine mud which 
 they dumped into the Chester embayment. The waves along the 
 shores sorted the materials and deposited them in different 
 places; the sands, which represented the coarser materials, settled 
 near shore ; the finer materials or muds were held in suspension, 
 were carried out to sea, and finally settled in the deeper waters 
 awav from the shores, to form the mud beds from which the 
 
POST-CHESTER VEGETATION 21 
 
 shales now conspicuous in the Chester series were developed. In 
 places in the deeper waters, conditions were favorable for the 
 existence of a wonderful variety of living animals, whose re- 
 mains were buried in the limy shale beds where their hard parts 
 were preserved and from which an abundance of fossils may 
 now be collected. In other places in this Chester sea, conditions 
 permitted the accumulation of pure limestones which can now 
 be seen in many of the exposures of Chester rocks. 
 
 Had the shoreline of the Chester embayment remained 
 stationary throughout Chester time, there would have been 
 deposited one great sandstone formation near shore, which 
 
 would have become finer and finer off shore, 
 The shifting merging into a shale formation composed of the 
 
 fine muds which had been held in suspension, and 
 still farther out from shore into a limestone. But such simple 
 conditions did not prevail. The shores of the embayment shifted 
 constantly, and the areas which at one time were submerged 
 became exposed because of the rising of the land or the lowering 
 of the sea level — only to be covered again by the sea because of 
 a reversal of conditions. At intervals the whole of southern 
 Illinois was above water, and the shoreline lay to the south 
 perhaps across what is now Kentucky. As a result of these 
 shiftings in the position of the shoreline, that portion of southern 
 Illinois where Chester rocks form the surface was successively 
 within the zones of sandstone, shale, and limestone formation, 
 and so the Chester strata are an alternating series of these dif- 
 ferent rocks. The Chester section gives evidence of no less than 
 eight shiftings in the shoreline. 
 
 After the final withdrawal of the Chester sea, the whole 
 of the southern portion of North America doubtless became 
 dry land and remained as dry land for a long time. A rich 
 Vegetation of ve £ etation sprang up, very different from what 
 post-Chester now covers the region. Instead of the usual 
 
 types of forest trees and herbaceous and shrubbj 
 
28 GEOLOGIC MAKING OF SOUTHERN ILLINOIS 
 
 plants with which we are familiar, a large number of the 
 trees and other plants were fern-like in character ; others 
 were gigantic relatives of the little club mosses of the northern 
 portion of our country which now grow to a height of only a 
 few inches and are not found at all in southern Illinois. Others 
 were great tree-like relatives of the horsetails which now grow 
 as slender plants only a few feet high at most, and still others 
 were cone-bearing trees with long and broad grass-like leaves. 
 Nowhere in all these forests was there a single representative 
 of the plants bearing showy and sweet-scented flowers which 
 make our own landscapes so beautiful. 
 
 The air-breathing animals of these forests were also strange 
 forms when compared with those we know today. Doubtless 
 there were numerous forms related to our living salamanders, 
 A . ...- f lizard-like forms who spent their early life 
 post-Chester stages in the waters of ponds and swamps, much 
 times as £ ots t h e tadpole which is the early stage 
 
 of the toad or frog. Perhaps there were a few air-breath- 
 ing reptiles, lizard-like creatures, which lived on land. 
 There may have been some land snails and fresh water snails 
 and clams. Flying about in these forests were many extraord- 
 inarily large insects, of the cockroach and dragonfly type for 
 the most part. One dragonfly with a wing spread of thirty 
 inches is known, and some of the cockroaches which lived dur- 
 ing this period were more than a foot long. 
 
 This period was the beginning of what we call Pennsyl- 
 vanian time. Dry land then prevailed in this section of the 
 continent, but to the east — along the border of old Appalachia 
 from Pennsylvania southward to Georgia — great 
 beginning* of quantities of terrestrial deposits of coarse sands 
 Pennsyl- and conglomerates and some coal were accumu- 
 
 lating. To the southwest, in Oklahoma and be- 
 yond, aquatic sediments, in part marine, were being deposited. 
 
ORIGIN OF THE ILLINOIS COAL BASIN 29 
 
 How long this dry-land condition continued in Illinois 
 cannot be measured in years. Popularly speaking, the time was 
 long, and even from the standpoint of geology it was a notable 
 
 interval. During this time, the land surface was 
 Pottsville undergoing slow erosion ; since the region was not 
 
 highly elevated above sea level, the erosion was 
 not as vigorous as in more mountainous regions. Large river 
 systems were developed, and some hundreds of feet of the older 
 sediments were removed. 
 
 Later a sinking of the land resulted in a great wash of 
 terrestrial sediments overspreading the region. These consisted 
 of coarse sands and conglomerates, which filled the depressions 
 in the land surface, and some finer sands and sandy shales. In 
 some places conditions were favorable for the accumulation of 
 vegetable matter in swamps; the peat w T hich was then formed 
 later became coal. 
 
 These sediments furnish the record of events in the Potts- 
 ville epoch of early Pennsylvanian time, and their remnants now 
 form the crest of the so-called Ozark ridge which extends east 
 and west across the State south of Harrisburg (fig. 1). At 
 Harrisburg, these same beds are far beneath the surface, because 
 of their northward dip brought about by later deformation in 
 the region, but they were originally laid down in an essentially 
 horizontal position. 
 
 After a time, the region which extended south beyond 
 
 Ohio River, north into northern Illinois, and west across Iowa, 
 
 Missouri, and Arkansas to Kansas and Oklahoma, became 
 
 ^ . . f stabilized at a level not much higher than that 
 Origin of 
 
 Illinois coal of the ocean. Upon this low, flat expanse the 
 basin drainage became more or less stagnant and great 
 
 swamps came into existence (fig. 7), some of them of great 
 extent, in which the abundant remains of vegetation slowly 
 
30 
 
 GEOLOGIC MAKING OF SOUTHERN ILLINOIS 
 
 collected. The conditions were such that this vegetable 
 matter did not decay readily, and it accumulated first as 
 great beds of peat. Occasionally quantities of sand and 
 mud were washed into the basin probably as a result of slight 
 elevations of lands to the north, and the peat beds were thus 
 buried under strata of sandstone and shale. At other times, 
 marine waters came in and spread over the area and lime- 
 
 Fig. 7. — Reconstruction of swamp forest in Pennsylvanian time. 
 After H. Potonie. 
 
 stone beds, some nearly pure and others more or less impure, 
 all carrying characteristic marine shells, were formed in the 
 basin. Again, muds were deposited under marine conditions, a 
 fact of which the presence of marine shells is an evidence. Such 
 conditions were followed by a recurrence of swamps in which 
 more peat beds were formed. 
 
 The result of these successive changes w T as the building up 
 of a series of sandstones, shales, coals, and limestones which 
 
ILLINOIS OZARKS 31 
 
 constitute the section now found in the Illinois coal basin. The 
 great weight and pressure of the superimposed strata upon 
 the peat beds caused these beds to be transformed gradually into 
 the coal which is the important mineral resource of southern 
 Illinois. 
 
 During this period of accumulation, the sediments were 
 laid down in an approximately horizontal position, and the coal 
 beds of Illinois and of western Kentucky were continuous. Yet 
 today the Illinois fields are separated from the Kentucky fields, 
 and between them are found rocks which are much older than 
 the coal-bearing strata. Obviously, some deformation in the 
 earth's crust was responsible for the elevation of these older 
 rocks above the coals. To the geologist comes the problem of 
 finding an explanation for the condition which exists; why was 
 this portion of the earth's crust deformed, and when did the 
 deformation take place? 
 
 A significant fact concerning the uplifted area in southern 
 
 Illinois is its position directly east of the Ozark region of 
 
 Missouri, which we know was uplifted repeatedly during 
 
 Paleozoic and later geologic times. It is not 
 Relation of ,., , , . ... , T11 . , , 
 
 uplifted area unlikely that the uplift in Illinois was due, in 
 
 to Missouri part at least, to the proximity of the area to the 
 ^)zarks 
 
 Missouri Ozarkia and its connection with it; so 
 
 we may very properly speak of the highlands of Illinois as a 
 continuation of the Ozarks. That the connection is popularly 
 recognized is shown by the fact that a small town in southern 
 Illinois bears the name Ozark. 
 
 The present elevation of the Illinois Ozarks is by no 
 means equal to their original height. Perhaps more than a 
 thousand feet have been removed from the tops of these hills 
 by erosion, and, although they could never have been really 
 high mountains, yet they must have been a much more formid- 
 able range than they are now . Their present elevation is due to 
 
32 GEOLOGIC MAKING OF SOUTHERN ILLINOIS 
 
 the facts not only that the rocks were bowed up in the past, 
 but also that the older rocks which have been uncovered 
 are harder and more resistant to erosion than the rocks to 
 the north. Consequently they remain as high parts simply be- 
 cause they are not easily worn away. Had they been softer 
 than the younger rocks, a depression or valley instead of a range 
 of rugged hills would mark the belt of the uplift. 
 
 In the eastern portion of the elevated belt across southern 
 Illinois, other factors besides proximity to the Missouri Ozarks 
 had their influence. One of these was the intrusion of igneous 
 Igneous material into the earth's crust which caused the 
 
 intrusion strata to dome up, as in the case of Hicks Dome. 
 
 As already stated, we now find limestones of middle Devonian 
 age exposed at the surface in the center of the dome, and if all 
 the strata which erosion has removed could be replaced, it 
 would be found that the doming was sufficient to form a 
 mountain which must have towered above its surroundings. 
 
 The molten material was injected also into cracks and 
 fissures in the existing strata, and formed thin dikes (fig. 8) 
 which have been found in coal mines near Harrisburg, and at 
 many other localities between Harrisburg and Princeton, Ken- 
 tucky. These dikes are all similar in character and must be 
 connected at some unknown depth with a great mass of igneous 
 rock which underlies the region. But the depth of this igneous 
 rock mass may never be known unless some extremely deep 
 drilling should be undertaken at a future time. 
 
 The injection of the igneous material led to another 
 
 change, namely, faulting. The phenomenon of faulting involves 
 
 the formation of more or less vertical or oblique cracks in the 
 
 earth's crust, extending to unknown depths, and 
 
 Faulting t | ie s j; p pj n g f t h e roc k strata on either side of 
 
 the cracks vertically, or horizontally, or both. Such dislo- 
 cations in the rock strata are known as faults. Tn all faults 
 
FAULTING 
 
 33 
 
 except where the motion is entirely horizontal, the rock strata 
 on one side of a fault are raised relative to the beds on the 
 opposite side ; the side which is raised is known as the upthrow 
 side, and the opposite as the downthrow side of the fault. 
 Faulting may be produced by the exertion of a tension or 
 stretching force in the earth's crust. When such a force is 
 exerted upon the rocks, many irregular cracks are likely to be 
 
 Dike 
 
 
 1 -|||||^ L 
 
 KrWJf - 1 
 
 
 
 . 
 
 \M 
 
 MUhRmK^H 
 
 Fig. 8. Dike in a mine in Saline County. 
 
 formed, the major ones of which will have a general direction 
 at right angles to the direction of the tension force. The rocks 
 will give way in the weakest place, which may be a belt of 
 considerable width, in which the cracks may outline a regular 
 mosaic of blocks of various sizes and shapes. The blocks so 
 formed will slip downward and may tip upon each other and 
 
34 
 
 GEOLOGIC MAKING OF SOUTHERN ILLINOIS 
 
 finally come to rest when the tension force has been relieved. 
 (Fig. 9.) So in this area, as the beds were domed up, the 
 inelastic crust was forced to stretch and innumerable cracks 
 formed. The wedge-shaped blocks which were formed slipped 
 and slid on each other and by their adjustment they took up 
 the slack produced by the stretching. The readjustments along 
 these fractures form the complicated system of faults which 
 exists in southeastern Illinois and western Kentucky. Along 
 some faults the dislocation has been as great as 2000 feet, else- 
 where only a few feet. 
 
 Hanging wall 
 
 Foot wall 
 
 Fig. 9. Diagram to show the movement of rock strata in normal 
 faulting. The hanging wall moved down with respect to the 
 foot wall. Bed A was continuous before the faulting took 
 place. 
 
 The phenomena of igneous intrusion and faulting are be- 
 lieved to be responsible for the accumulation of the famous 
 fluorspar deposits of Illinois and Kentucky. (Fig. 10.) Igneous 
 rocks are known to be a source of fluorine and 
 
 Fluorspar most of the important ore bodies in the district 
 
 deposits . 
 
 are found in veins located along tault lines. In 
 
 a few places the fluorspar is not associated directly with a 
 fault, but it is near some fault to which it is doubtless geneti- 
 cally related. 
 
II MK 01' DEFORMATION 
 
 35 
 
 The determination of the time when all the deformation 
 and accompanying phenomena took place is a problem for the 
 geologist. We know certainly that it occurred after the forma- 
 tion of the Pennsylvanian rocks for they are 
 
 Time of _ involved in the deformation. In the extreme 
 
 deformation . . 
 
 southern portion or Illinois are Cretaceous strata, 
 
 late Mesozoic in age, which, so far as we know, are not 
 
 deformed, but the deformed older rocks pass beneath them. 
 
 With these data, which are all we have at the present 
 
 time, we can only say that the disturbance took place 
 
 Fig. 10. — A fluorspar mine in southern Illinois. 
 
 after the last of the Pennsylvanian rocks were formed and 
 before the deposition of the Cretaceous rocks, but this interval 
 represents millions of years. The fault movements were prob- 
 ably distributed over a long period of time, for no geologist 
 believes that they can be instantaneous. It is altogether prob- 
 able that slight movements are still taking place along some of 
 the fault lines. Scarcely a year passes without the recording 
 of one or more slight earthquake shocks in this portion of Ohio 
 
36 GEOLOGIC MAKING OF SOUTHERN ILLINOIS 
 
 Valley, which are more than likely the results of settlement 
 along some of the previous lines of deformation. A little more 
 than 100 years ago a severe earthquake occurred in the middle 
 Mississippi Valley which may have been caused by an adjust- 
 ment of greater magnitude than usual along one of the frac- 
 ture lines. It may be asserted with certainty that during the 
 active period of all this deformation, frequent earthquakes of 
 great severity shook southern Illinois. 
 
 It is not at all unlikely that a preliminary uplift of the 
 whole Illinois Ozark region, associated with adjustments in the 
 Missouri Ozarkia and not accompanied by notable faulting, 
 occurred long before the igneous intrusion and complex faulting 
 in the eastern portion of the region. All of the facts are not yet 
 known, but every season of observation adds to our under- 
 standing. 
 
 There is no evidence that the sea ever entered Illinois 
 
 north of the uplift after the deformation. The region remained 
 
 as an extended land area for an inconceivably long time. 
 
 , Throughout Mesozoic time it doubtless was the 
 Mesozoic and 
 Cenozoic home of successive generations of gigantic rep- 
 
 conditions t ji es wn j cn are known to have inhabited North 
 America during this time (fig. 11), and these creatures were 
 doubtless followed by the great mammalian hordes that charac- 
 terized Cenozoic time. Through these millions of years, the 
 vegetation of the region also underwent profound changes, but 
 none of the record is preserved. We know little of the topo- 
 graphic features of the land during this time — whether it was a 
 plains or hill country — and little of its drainage; but it is likely 
 that the relief was much greater than that of our Illinois 
 prairies. 
 
 In Cretaceous time, an arm of the sea did reach into the 
 southern tip of Illinois from the Gulf of Mexico, but the ab- 
 sence of any typical marine fossils in the beds may mean that 
 
MESOZOIC AND CENOZOIC TIME 
 
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38 GEOLOGIC MAKING OF SOUTHERN ILLINOIS 
 
 typical marine conditions never existed. There are also widely 
 
 distributed deposits in the south, probably Tertiary in age, but 
 
 it is not likely that they are marine. They were formed when 
 
 topographic conditions were far different from those of today, 
 
 long before Ohio River and its tributaries were a part of the 
 
 landscape. 
 
 With the coming of the Pleistocene time and the great ice 
 
 age, glaciers came down from the north, one of which reached 
 
 the vicinity of Harrisburg. These destroyed or drove out all 
 
 animal and vegetable life, and their deposits 
 
 Pleistocene filled the depressions in the surface, leaving it 
 time F . 
 
 approximately as we know it today. 
 
 We look backward upon the many millions of years during 
 which all of these events have come about. When we realize 
 that the entrance of man into the story is the very latest event 
 of all, we can only be amazed and overwhelmed by a glance 
 into the future. There is every reason to predict that the con- 
 tinuation of this history into the future may be as long as the 
 time which has passed. Man is a mere infant; he has taken 
 only the first step in his career, but he has learned to control 
 his environment as no other animal has ever done. The law of 
 progress is manifest, and we cannot help feeling that in the 
 distant future our descendants will look back upon us in much 
 the same manner as we now look back upon the men of the 
 Old Stone age. 
 
GLOSSARY 
 
 Brachiopods — Marine, invertebrate shell-animals, like those com- 
 monly known as "lamp shells". They have two "shells" or valves 
 which are different, one from the other, but are bilaterally 
 symmetrical. See fig. 6, p. 25. 
 
 Cephalopod — Marine, single shelled invertebrate animal, whose shell 
 may be either straight or more or less coiled in one plane and 
 consists of several separate chambers. The pearly nautilus is a 
 living example of this type of animal. See fig. 6, p. 25. 
 
 Conglomerate — A rock formed by the cementation of gravel. 
 
 Crinoid — Marine animal sometimes called "sea lily". It is attached 
 to the sea floor by a stalk and its upturned mouth is surrounded 
 by long, feathery arms. See fig. 6, p. 25. 
 
 Crustal — Pertaining to the outer portion of the earth, or that portion 
 whose nature is partly known from geological examinations. 
 
 Deformation — Warping, folding or any massive displacement of the 
 earth's crust. 
 
 Delta — Deposit of sediment at the mouth of a river. 
 
 Dike — A body of rock formed by the solidification of fluid rock 
 which was forced into a fissure. 
 
 Embayment — A considerable portion of shallow ocean water upon the 
 continent. 
 
 Emergence — A change in the relations of land and ocean resulting in 
 the conversion of the ocean bed into land. 
 
 Era — A major subdivision of geologic time. 
 
 Erosion — Wearing away. Erosion is the term applied to all the 
 processes by which earthy matter or rock is loosened and re- 
 moved from one place to another. 
 
 Evolution — A term usually applied to those changes which animal 
 and plant life have undergone in the history of the earth, result- 
 ing in the origin of new and varied forms. 
 
 Fault — A displacement along a fracture interrupting the original 
 continuity of a rock formation. 
 
 Fauna — A natural assemblage of animals inhabiting a given area, or 
 existing within a certain period. 
 
 39 
 
40 GLOSSARY— Continued 
 
 Fluorspar — A mineral composed of calcium and fluorine, usually 
 
 transparent and purplish or pinkish in color. 
 Fossil — Any remains, impression, or trace of an animal or plant of 
 
 past geological times which have been preserved in rocks. 
 Gastropod — A single-shelled invertebrate animal, whose shell, if 
 
 coiled, is conical in shape. Snails and slugs are living examples. 
 
 See fig. 6, p. 25. 
 Glacier — A field of ice of such thickness that it moves outward from 
 
 the center of accumulation. 
 Historical time — The time covered by human history as recorded by 
 
 actual inscriptions and writings. 
 Intrusion — A mass of igneous rock which, while molten, was forced 
 
 into or between other rocks. 
 Invertebrate — An animal which does not possess a spinal column. 
 Larval — Pertaining to an early stage in the development of some 
 
 animals. The caterpillar or worm which eventually becomes a 
 
 butterfly is a typical example. 
 Limestone — A sedimentary rock composed of consolidated lime mud 
 
 or fossil shells, or both. 
 Noachian Deluge — The great flood of biblical fame. 
 Outcrop — Exposure of rock. 
 
 Peat — Accumulation of vegetal material under swamp conditions. 
 Pelecypods — Two-valved shell animals, such as clams, scallops, 
 
 oysters, and mussels. The two shells are mutually symmetrical, 
 
 but there is no symmetry in either shell. See fig. 6, p. 25. 
 Period— A major sub-division of a geologic era. 
 Relief (topographic relief)— The difference in elevation between the 
 
 highest and lowest points of an area. 
 Residuum — Final products of rock decomposition in place. 
 Sandstone — A rock formed by the cementation of sand grains. 
 Sedimentary rocks — Rocks formed by the accumulation of sediment 
 
 either in water or in air. 
 Shale — A rock formed by the compacting or cementation of clay 
 
 or mud. 
 Spore — The reproductive body of flowerless plants. Spores are 
 
 minute germs which have the same function as seeds. 
 
GLOSSARY— Concluded 41 
 
 Stratigraphic — Pertaining to the order and relative position of the 
 rocks of the earth's crust. For example, in Saline County the 
 Harrisburg coal bed lies stratigraphically below the Herrin coal 
 bed. 
 
 Stratum — A layer of rock. 
 
 Structure — The position or attitude of rock strata with reference to a 
 horizontal plane. For example, rock strata have a horizontal 
 structure if they lie horizontally. 
 
 Submergence — A change in the relation of land and ocean whereby 
 a former land area is covered by the sea. 
 
 Surfkial deposits — Unconsolidated material lying on bedrock. 
 
 Trilobite — An extinct marine animal which resembles the crabs, 
 crayfish, and lobsters. See fig. 6, p. 25.