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by John P. Bluemle North Dakota Geologicol Survey and Lee. Clayton Wisconsin Geological Survey
Educational Series 14 North Dakoto Geological Su vey Don L. Halvorson, State Geologist 1982 Era tem
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This chart shows the changes that have taken place 10 the geology and livmg things in orth Dakota and nearby areas ver the last 600 million year . MAIN ROCK TYPE PER 00 IN NORTH DAKOT~ u Glacial Sediment o N o Sandstone I s tstane Z shale, lignite 50 This generahzed geologic column of I.LI orlh Dakota IS drawn with a vertIcal U hnear tIme scale of about 80 million years --...... ---4----- +__ 67 to the IDch. It rerresents only the Ia t 600 mllhon years 0 the earth's hIstory. To Loyersofsi t,clay,and 100 Cretaceous sand neor the top; shale 'n IDclude all of the time that has elapsed C sIDce the formatIon of the earth, the ":J the middle i sand at the N base column woulc1have to be e tended down u 1---....;...--+------+-140 ward another 4 feet. It would have to be 16 o 150 extended to a total length of about N feet If It were to include all of the time o IDee rhe universe formed (fig I). (f) Juross c Sho e ,lIrnestone t and s· tstone ; some redoeds The age gIven for the boundanes w and evopor tes ~ between each of the geologic pertods ~-.,---4------4-200 200 (e.g.' the Cretaceou began 140 mtlhon years ago) are based on the late t available Sho e, siltstone I TrKlss c sands one I red beds and e timate ( mos Salvadore. The Umversity evaporites of T at ustin, for American SSOCI alion of Petroleum Geologl ts. CO U A ~---+------+-250 250 Project ovember 1981). L mes one, evaporlles The era and equence column how do om te and sha e those periods of time that are repr nted by pre erved sedimentary rode sectIon in orth Dakota; the e are shown a the darkened part of the column (Sauk Tippecanoe. etc.). The blank part of the column between t e named sedImentary sequence, IDdlcate pertods of tIme not 350 represented by a rock record In orth Dakota. The lack of rocks of the e ages I due euher to nondeposItion or the removal by erosIon of rocks that may u once have been present. Less than half of 400 o geologic time IDce the beginning of the N I Cambrran I represented by a pre erved o rock sectIon ID orth Dakota. The mo t W ..J t complete reco is found 10 the central ~ Precllmbr on 19neolis or:d metamor~h c crys'a one rOCl .....----~""----...... ---...... ------.L--~4500 GEOLOGIC TIME IN NORTH DAKOTA by John P. Bluemle North Dakota Geological Survey and Lee Clayton Wisconsin Geological Survey EDUCATIONAL SERIES 14 NORTH DAKOTA GEOLOGICAL SURVEY Don L. Halvorson, State Geologist 1982 Printed by Richtman's Midwest, Fargo, NO 58107 1982 CONTENTS Page GEOLOGIC TIME IN NORTH DAKOTA 1 Introduction 1 The Length of Geologic Time 1 NOTES .. 2 GLOSSARY 15 Geologic Time Terminology 15 Explanation of Some of the Terms used on the Time-Flow Charts . 16 ILLUSTRATIONS Figure Page 1. The past 15 billion years in North Dakota and the world . 6 2. The past 1.5 billion years in North Dakota and the world . 7 3. The past 150 million years in North Dakota and the world ..... 8 4. The past 15 million years in North Dakota and the world ..... 9 5. The past 1. 5 million years in North Dakota and the world ...... 10 6. The past 150 thousand years in North Dakota and the world ...... 11 7. The past 15 thousand years in North Dakota and the world ...... , 12 8. North Dakota's climatic history for the last 15,000 years 13 ii GEOLOGIC TIME IN NORTH DAKOTA south. Few people believed him, however, and his idea did not catch Introduction on. In the third century B . C. , The intent of this publication is to Eratosthenes depicted a spherical earth give North Dakotans a better concept and even calculated its diameter and of geologic time. I have attempted to circumference, but the concept of a show the immense amount of time it has spherical earth was beyond the imagi taken for our state to become the place nation of most men. Less than 500 it is today. A series of time-flow years ago, sailors aboard the Santa charts (figs. 1 through 7) show events Maria begged Columbus to turn back that have occurred throughout the lest they sail off the earth's edge. earth's history. Each successive chart Similar opinions and prejudices about represents a tenth of the geologic time the nature of the earth have waxed shown on the preceding chart. Thus, and waned down through the centuries; the first chart is a very general however, most people appear to have accounting of the events that occurred traditionally believed the earth to be over the entire 15 billion-year history quite young, that its age might be of the universe, the second chart measured in terms of thousands of amplifies and details the last 1.5 billion years, but certainly not in millions. years on earth, etc. The final chart The evidence for an ancient earth (fig. 7) zeros in on the last 15,000 is found in the rock record, the years of North Dakota's history. materials that form the earth's crust Beyond that, geology becomes "trans and surface. The rocks and sediments formed" into recorded history, a study are not all the same age, or even I leave to archaeologists and historians. nearly so, but, like the pages in a Incidentally, the concept of using suc long and complicated history book, cessive time charts to show contem they record the earth-shaping events poraneous geologic events in various of the past; however, the record is parts of the world is borrowed from incomplete. Many "pages" are missing similar methods sometimes used by and many others are tattered, torn, or historians. difficult to decipher. Even so, enough North Dakota's climatic and vege of the record is preserved to allow tative history during and since the Ice geologists to determine that the earth Age, especially since the most recent is many millions of years old. glacial epoch, the Wisconsinan glacia Two time scales are used to date tion, is of particular interest to many geologic events and to measure the age people and this is treated in a little of the earth: a relative time scale, more detail. (fig. 8). based on the sequence of layering of the rocks and sediments; and an The Length of Geologic Time atomic time scale, based on the natural Geologists, like historians dealing radioactivity of chemical elements in with the development of civilization, some of the rocks. The relative scale need some method of relating important evolved along with the science of events to one another. Unlike the geology; the atomic scale is a recent historian, however, the geologist development borrowed from physicists thinks in terms of millions of years, and chemists and applied to geologic not years or centuries. Time is a problems. fundamental consideration in all geo At the close of the eighteenth logical research, but it is difficult for century, the haze of fantasy and many people to comprehend the immen mysticism that tended to obscure the sity of geologic time. true nature of the earth was being Early speculations about the nature swept away. Careful studies by geol of the earth inspired much of the lore ogists showed that rocks have diverse and legend of ancient civilizations. At origins. Some rock layers, containing times, ancient people had considerable clearly identifiable fossil remains of insight. The Greek historian, fish and other forms of aquatic animal Herodotus, in the fifth century B. C. , and plant life, originally formed in made one of the earliest recorded seawater. Other layers, consisting of geological observations. After finding sand grains winnowed clean by the fossil shells far inland in what are now pounding surf, formed as beach parts of Egypt and Libya, he correctly deposits that marked the shorelines of inferred that the Mediterranean Sea ancient seas. Some layers are in the had once extended much farther to the form of sand bars and gravel bands, which consist of rock debris spread University, published a list of geologic over the land by streams. Some rocks ages based on radioactivity. Although were once lava flows, or beds of cin Boltwood's ages have since been re ders and ash thrown out of ancient vised, they did show correctly that volcanoes; others are portions of large the duration of geologic time would masses of once-molten rock that cooled have to be measured in terms of mil very slowly beneath the earth's sur lions, not thousands of years. face. Still other rocks were so trans The next 40 years was a period of formed by heat and pressure during expanding research on the nature and the heaving and buckling of the earth's behavior of atoms, leading to the crust in periods of mountain building development of nuclear fission and that their original features are obliter fusion as energy sources. A by ated. Some sediments were deposited product of this atomic research has by glaciers, like those that once been the development and continuing flowed over North Dakota, or by the refinement of the various methods and lakes and streams that formed from the techniques used to measure the age of melting ice. the earth materials. The most precise From the results of studies on the dates have become available since 1950. origins of the various kinds of rocks A chemical element consists of (petrology), coupled with studies of atoms with a specific number of pro rock layering (stratigraphy), and the tons in their nuclei, but different evolution of life (paleontology), geol atomic weights, owing to variations in ogists reconstruct the sequence of the number of neutrons. Atoms of the events that have shaped the earth's same element with differing atomic surface. Their studies show, for weights are called isotopes. Radioactive example, that during a particular decay is a spontaneous process in episode, the land surface was raised in which an isotope (the parent) loses one part of the world to form high particles from its nucleus to form an plateaus and mountain ranges. Follow isotope of a new element (the ing the uplift of the land, the forces daughter) . The rate of decay is con of erosion attacked the highlands and veniently expressed in terms of an the eroded rock debris was trans isotope's half-life, or the time it takes ported and redeposited in the low for one-half of the nuclei in a sample lands. During the same interval of time to decay. Most radioactive isotopes in another part of the world, the land have rapid rates of decay (that is, surface subsided and was covered by short half-lives) and lose their radio the seas. With the sinking of the land activity within a few days or years. surface, sediments were deposited on Some isotopes, however, decay slowly, the ocean floor. In this way, sand, and several of these can be used as silt, and clay eroded from areas like atomic clocks. the North Dakota badlands by the Dating rocks by using these radio Little Missouri River were carried by active timekeepers is simple in theory, the Little Missouri, Missouri, and but the laboratory procedures are Mississippi Rivers to the Gulf of complex. The numbers of parent and Mexico, which has subsided to receive daughter isotopes in each specimen are the sediments. The evidence for the determined by various kinds of analyt existence of ancient mountain ranges ical methods. The principal difficulty lies in the nature of the eroded rock lies in precisely measuring extremely debris, and the evidence of the sea tiny amounts of isotopes. includes fossils that accumulated with At the present time, the the sea-bottom sediments. potassium-argon method is one of the The discovery of the natural most useful dating methods available to radioactive decay of uranium in 1896 the geologist because it can be used by Henry Becquerel, the French on rocks as young as a few thousand physicist, opened new vistas in many years as well as on the oldest rocks fields of science. In 1905, the British known. Potassium is found in most physicist, Lord Rutherford, after rock-forming minerals. The half-life of defining the structure of the atom, its radioactive isotope, potassium-40, made the first clear suggestion for is such that measurable quantities of using radioactivity as a tool for mea argon (daughter) have accumulated in suring geologic time directly. Shortly potassium-bearing minerals of nearly thereafter, in 1907, Professor B. B. all ages. The amounts of potassium and Boltwood, radiochemist at Yale argon isotopes can be measured accu 2 rately , even in very small quantities. chemical elements that make up the Where feasible, two or more methods of earth at about 6 billion years. The analysis are used on the same specimen reasoning behind this estimate involves or rock to confirm or verify the re theoretical concepts about the origin of sults. uranium, which stipulate that not more Another important atomic clock than half of the first-formed uranium used for dating purposes is based on consisted of the uranium-235 isotope. the radioactive decay of the isotope But, because of the changes in iso carbon-14, which has a half-life of topic composition that have taken place 5,730 years. Carbon-14 is being pro throughout geologic time as a result of duced continuously in the earth's radioactive decay, uranium today upper atmosphere as a result of nitro contains only about 0.7 percent of the gen-14 isotopes being struck by neu uranium-235 isotope. Calculations based trons that have their origin in cosmic on the known decay rates for these rays. This newly formed radioactive two isotopes indicate that a span of carbon becomes uniformly mixed with about 6 billion years is required for the nonradioactive carbon in the car the isotopic composition of uranium to bon dioxide of the air, and it eventu have changed from 50 percent to 0.7 ally finds its way into all living plants percent uranium-235. Had uranium and animals. In effect, all carbon in originally consisted of more than half living organisms contains a constant of the uranium-235 isotope, the calcu proportion of radioactive or carbon-14 . lated age of uranium would exceed 6 After the death of the organism, the billion years. amount of radioactive carbon gradually Another estimate of the earth's age decreases through radioactive decay as is based on the progressive build-up it reverts to nitrogen-14. By mea in the earth's crust of lead isotopes suring the amount of radioactivity that have been derived from radio remaining in organic materials, the active decay of uranium-238, uranium amount of carbon-14 in the materials 235, and thorium-232. Studies of the can be calculated and the time of the relative abundance of these radiogenic organism's death can be determined. leads suggest that the earth is not For example, if carbon from a sample older than 5.5 billion years. of wood is found to contain only half Finally, it is interesting to try to as much carbon-14 as that from a visualize just what 4 or 5 billion years living plant, the estimated age of the really means. Suppose that the earth old wood would be 5,730 years; if it is 5 billion years old and then suppose contained a fourth as much, the age that all of those 5 billion years could would be 11,460 years. be compressed into a single imaginary The radiocarbon clock has become year. At that scale, each second of an extremely useful tool in dating the our contemporary time would be equiV important episodes in the recent pre alent to 160 years of geologic time, history and history of man but, due to each day to 14 million years. Using the relatively short half-life of carbon this time scale, dinosaurs did not come 14, the clock can be used for dating on the scene until mid-December, and events that have taken place only then they lasted six days. The Ice within about the past 50,000 years. Age began in North Dakota at about Some of the oldest known rocks 6: 45 p.m. on December 31 and ended found in North America are in south only one minute before midnight. western Minnesota. The Morton Gneiss Primitive man arrived on earth between appears to have formed about 3~ billion 10 and 10:30 p.m. in the midst of the years ago. Other rocks with similar Ice Age. At 13 seconds before mid ages occur in South Africa, Finland, night, Christ was born, and at 6 and Australia. Estimates of the age of seconds, Bjarni Herjolfsson discovered the earth are based on the decay rates America. * The United States of of radioactive isotopes. One such America, now over two centuries old, estimate is based on two long-lived has existed for less than two geologic isotopes, uranium-238 and uranium seconds. 235. It places the maximum age of the 3 NOTES *At the risk of starting an Italian/Scandinavian feud, I will point out that Bjarni Herjolfsson (a Viking) discovered (but did not land on) the North American continent in the year 985 or 986. He told Leif Ericson about it, and Ericson was the first European to set foot in North America. In 1492, Christopher Columbus dis covered the West Indian Islands, and in 1499 he was the first person to land in South America (if Amerigo Vespucci didn't get there first, perhaps in 1498 or 1499). John Cabot discovered North America in 1497. Assorted Welshmen and Phoenicians have also been credited with the European discovery of the New World, but these are not documented. Clovis Man (and woman) entered Alaska from Asia sometime during the Wisconsinan Epoch (earlier than 12,000 years ago) at a time when the Bering Strait was a land bridge due to the lowered sea level resulting from glaciation. Clovis Man migrated southward into North Dakota about 11,700 years ago, as soon as the glacier receded away from the Rocky Mountain front, allowing easy passage south ward. These people were the ancestors of the Native American population that lived throughout the Americas when Europeans arrived much later. 4 Figures 1 through 8 The Time Charts 5 Measured geologic o time . Archaeology EARLIEST GOOD GEOLOGIC RECORD ~ INlf ... FIRST ABUNDANT FOSSILS Qll:l. 1 billion about 600 million years !:l'" ! llIl 14QI t (JJ ii: .s z eX: First fossil photosyntheticI plants dated ~ I e::: J radiometrically at ab,ut 2.9 billion years OJ :::E I eX: u Water precipitates from atmosphere and w ~ e::: oceans form about 3.0 billion years ago I c.. ~ I co Formation of the Earth about >-:l 4.5 billion years ago § 5 bill-ion ,- I Solar system originates I - ? 5.9 billion years ago? I I I ? Milky Way originates ~ 9 billion years ago co - >-:lco I ~ 10 billion ? 8 1 ? ~ co >-:lco ~ E-< ? "BIG BANG"--primordial nucleus explodes? 15 billion I (or whatever theory you prefer) I Figure \. The past 15 billion years in North Dakota and the world_ 6 Measured Geologic Time North Dakota Global Marine Episode Geologic Worldwide Paleontology Eras Periods S aded Event 0 uaternar glaciation mammals-primates-grasses Tertiary erosion extinction of dinosaurs thick deposits of u Cretaceous shale first mammals 0 red beds dinosaurs dominate N Jurassic 0 first birds V) erosion w Triassic ::E early dinosaurs red beds ...... Permian ..... large amphibians o '"Q) u early repti les Pennsylvanian "-'~ o ;:l u Mississippian o insects--much plant life carbonates first amphibians ....,'">:: Q) ~ Devonian Q) ~ o a", I w 0"" -' carbonates ~ ~II first land plants 0:( c deposited ~ primitive fishes 500,000,000 Williston Basin~ ! trilobites--algae Cambrian starts to form first abundant fossils I Z early worms W V) w 0::: c- w 0::: o I..L.. '" W u c:l "'" 0 ~ V) 0::: u 0:( ..... w ..c:: > Po< ~ 0 <= S 1,000,000,000 ....,'" '" Q) green algae ' .Q S E ~ u'" >:: Q) ' '" c- ;:l'" 0 Q) >:: first known animal .....~ (jellYfish) >...... >:: S'" 1,500,000,000 Figure 2. The past 1.5 billion years in North Dakota and the world. 7 o North Dakota Western North America ·Plei t n Coleharbor Group § g&ac&at&on Pliocene Charging Eagle Fm. S OJ ~ <= F~~~~i;:':~; ~ t I I I o Miocene OJ I z i aooling climate unconformity ...... U1:(l! ..., '" '" most of North Dakota Brule Fm. ~ .~ subjected to erosion u ...... ~~ o Oligocene Chadron Fm. N o ~ ~I' o 0 ~ .r-! .r-!' OJ f------1 major u <= ~ ~i OJ unconformity tn C; C;I o Eocene 1 OJ ~ ;1 50,000,000 Golden Valley Fm. Sentinel Butte Fm. ,''' f------1Bullion Creek Fm. I ~ 0 lignite deposited >. '" I Slope Fm. 1 z Paleocene Cannonball Fm. § ~ w Ludlow Fm. :so i Cll Rocky Mountains formed Vl ~ ~ W f--+--l-----IHell Creek Fm. tl I 0:: triceratops common " 0) ~I~ Fox Hills Fm. ~ '"I W 1 0:: @ 8 2 o ~I~ ~ LL ;; I W Pierre Fm. --' I'''' '" I 0::> ~'12 o~ Vl 0:: <.H I ~ c:( W > Niobrara Fm. .il'o~ ~ black shale V1 ~ Carlile Fm. ..., OJ :=l U ~ deposited in o <::: Cll OJ OJ .r-1 ::; ~ seas throughout 100,000,000 u Greenhorn Fm. ..., <:l"-' North Dakota ~ ~ ,'" u +-''" ,'" ;..:> OJ >0. o " OJ Cll N U ;..:> ::.. o Belle Fourche Fm. ~ sand and shale deposited Vl ~ w Dakota Group <--, as sea floods ::E: Nort.h Dakota ! I I Ellis i Jurassic Group I sea drains from North Dakota 150,000,000 Figure 3. The past 150 million years in North Dakota and the world. 8 ~ ------~------...... (J'1 (J'1 .a . a a a a a a a .a .a a a a a a a a a a a MIOCENE PLIOCENE PLEISTOCENE Early I Middle Late Early Early ate I Late 3: I 11 I 10 I 9 I 8 I 7 I 6 5 . 62 "'"m glacial epochs ----- interrupted by warm periods I - Figure 5 (next page) ...... U1 a U1 a a a a .a .a .a a a a a a a a a a PLEISTOCENE EPOCH :E tJ)~·O (TI r :".tJ) I:l> I:l> -s rl" ~(') 0 ~ro :. 0:::>ro (') '< I:::> ro -vlduvai Matuyama reversed "'"' Jaramill a Bruhnes "T'l magnetic event magnetic event normal ." ." 00' ~ Milankovitch Curve warmt~ ;n Cold ; (1) BI:lncan I Irvingtonian fRanc ho I eb rean "0 -6" rt».,; '0 ; ~ til (1) I" 'io ::rl" tort».,; tJl 'i (1) • 'i t;;.~ ~ '" ..... ::r rt >< re o ;oj (1) ..... U. .... ~ CfJ man uses fire ::r (1) • 'i 'i >-'t-t> ~ s rt 0 >-' ...... 0 rl ..... ~ rt in Old World '00 (1) 0...... (1)0 'i (1) 0.. l-1 ..... >-' (l) rt ~ rt tJl I" Po. ;oj » rt til (1) >-' '< .... ::r ...... (1)., a Homo erectus Homo sapiens 0 ;;: (1) til S' o Z t-t> 0 I)Q a~ternating periods of co~d (g~aciation) and warm c~imate ~ >-' ------. I" .,0 n ~~ ~ ;>I" .... ~ 0 I" e Ol rt (1l l.LJ l 14,000 e.: -J 15,000 Figure 7. The past IS thousand years in North Dakota and the world. 12 ,,~ "'------ ~---,------,r-~__r-r_i ----,-,------,.--,-----r---->~__,____- -G> o~ «§eU"" c:&~ °e ~~ ... medium ." , ~ .... '" u; :I: Vl -f ~ gross Figure 8. North Dakota's climatic history for the last 15,000 years, This is a time-distance diagram, time represented in a vertical direction, and distance, from southwestern to northeastern North Dakota, shown from left to right. The diagram illustrates when certain climatic events occurred in various parts of the state_ For example, as conditions dried about 9,000 years ago, short-grass prairie first covered southwestern North Dakota (about 8,700 years ago) and advanced northeastward, although not into the easternmost part of the state, before giving way to medium-grass prairie over most of the state about 5,000 years ago as the climate again became mOre moist. The Lockhart, Moorhead, and Emerson Phases, shown in eastern North Dakota between about 13,000 and 9,500 years ago, portray the history of Lake Agassiz. The Burnstad advance, and other advances, represent glacial advances of the last glacier that affected North Dakota, The "stagnant glacial ice" occurred mainly on the Missouri Coteau between about 14,000 and 9,000 years ago (it also was widespread over the Turtle Mountains and Prairie Coteau in southeastern North Dakota), The four columns on the right (A, B, C, and D) represent, in a general sort of way, how various specific conditions changed over the past 15,000 years, Column A shows how the mean annual temperature varied; B shows how precipitation varied; C shows the relative amount of slope-wash erosion from the hillslopes; and D shows the amount of wind-blown sediment being deposited on gentle slopes and in low areas. In all four columns, the increasing amount is to the right, decreasing amount is to the left. ' 13 Most of what we know about past climatic conditions (fig. 8) we have learned from studies of pollen, and more recently, beetle assemblages, found in the sediments of lakes and bogs. Particularly valuable research is currently being done along these lines at the Fossil Beetle Laboratory at North Dakota State University in Fargo. The first three points are based on studies made beyond the limit of the glaciation in central Iowa: 1. Throughout Wisconsinan time: (70, 000 to 10,000 years ago) Summers were generally more cool and moist over the midcontinent region than they are today; during much of this time, most of North Dakota, except for the southwest corner, was glaciated. 2. During Late Wisconsinan time: (25,000 to 10,000 years ago) The coldest conditions occurred about 17,000 years ago--at that time, central Iowa (at the edge of the glacier that covered North Dakota) had summer temperatures comparable to those at the treeline in Canada today, (a mean July temperature of about lOoC, compared to the 23°C of today (50°F vs. 73°F). North Dakota was heavily glaciated at that time and it is not known what the average temperature was here--certainly considerably colder than Iowa at that time. 3. By 13,000 years ago, summer temperatures in North Dakota had warmed by several degrees and they were comparable to those in the mid-Boreal Forest today. North Dakota was being deglaciated so we may assume that the average annual temperature was above freezing; however, changes in both annual temperature and annual precipitation have to be considered in determining when and at what rate deglaciation occurred. The remaining points are based on studies made in bogs and sloughs in North Dakota, northern Minnesota, and Manitoba: 4. From 13,000 until 9, 000 years ago, summer temperatures warmed slightly to those at the southern edge of the Boreal Forest today (fig. 8). The warm ing may have occurred gradually, or it may have been in steps--we don't know which. North Dakota became wooded as the ice melted away (mainly spruce-poplar woodland). 5. About 10, 000 or 9,500 years ago, conditions became much drier (fig. 8) so that prairie grassland replaced much of the woodland in North Dakota, first in the west, then farther east. Temperatures also warmed. 6. Warming and drying continued, reaching a maximum between about 7,000 and 5,000 years ago (the time known as "hypsithermal" or "altithermal"). At that time, North Dakota was considerably drier and warmer than it is today (fig. 8). 7. Since about 3,500 years ago, the climatic patterns have been similar to today, although variations that may last for several hundred years at a time do occur. 14 GLOSSARY three-fold division of rocks overlying the Paleozoic rocks Geologic Time Terminology in Germany IPrecambrian: mainly complex metamor Jurassic: refers to rocks of that phic and igneous rock that makes age outcropping in the Jura up most of the volume of the Mountains in Germany earth's crust; very few fossils; Precambrian time makes up about Cretaceous: a Latin word meaning 90% of geologic time. "chalk"; refers to the chalk formations in France and Paleozoic Era: "ancient life" England Cambrian: a Latin word meaning Cenozoic Era: "recent life" "Wales" Tertiary: an old term held over Ordovician: a word derived from from early attempts to sub "Ordovices." an ancient tribe of divide the geologic record into Wales three divisions ("Primary. " "Secondary." and "Tertiary"); Silurian: a word derived from the first two terms have been "Silures," a tribe of Wales discontinued. Carboniferous: "coal-bearing"; in Quaternary: refers to the very the United States: recent deposits that contain fossils which contain living "Pennsylvanian" (from the representatives state of Pennsylvania where coal of this age is mined) and We also use seven epochs. subdivisions "Mississippian" from the upper of Tertiary and Quaternary time; they Mississippi Valley (Iowa are based on the relationships between Missouri- Illinois) life forms: Permian: a word referring to rocks Paleocene: "ancient recent" exposed over much of the province Eocene: "dawn of recent" of Perm, Russia, just west of the Oligocene: "little recent" Ural Mountains Miocene: "less recent" Pliocene: "more recent" Mesozoic Era: "middle life" Pleistocene: "most recent" Holocene: "recent" Triassic: a term that refers to the 1 Explanation of Some of the Milankovitch curve. Originally pro Terms Used on the Time-Flow Charts posed by the Yugoslav scientist, M. Milankovitch, the Milankovitch curve is Magnetic polarity. Recent studies of based on the premise that both the the magnetic properties of numerous orbit and axial rotation of the earth samples of basalt from many parts of vary through geologic time in a the world demonstrate that the earth's systematic way. The variations result magnetic field has been reversed many in changes in the seasonal and lati times during the last 70 to 80 million tudinal disturbances of the earth's years. That is, there have been insolation (the way it holds and re periods of 1 to 3 million years during flects the heat it receives from the which the magnetic poles have been sun). The Milankovitch curve considers close to their present location, followed three main factors: 1) variations in the by similar periods during which the eccentricity of the earth's orbit north and south poles have been through time--the period is 96,000 reversed. At least nine reversals have years and it results in about a 25% occurred in the last 4.5 million years. difference in the earth's heat The present period of "normal" reflecting ability; 2) the "precession of (Bruhnes normal on figures 4 and 5) the equinoxes"--the period is 21,000 polarity began about 700 ,000 years ago years; today the earth is nearest the and was preceded by a period of sun in January; in 10,000 years it will reversed polarity (the Matuyama re be nearest in June; 3) the oblicity of versed on figures 4 and 5) beginning the ecliptic or the angle of the earth's 2.5 million years ago (two short orbit--the period is 41, 000 years; the periods of normal polarity occurred angle is 23.5° and the variation is within the Matuyama reversed period). about 2.5° but there is evidence that The major intervals of alternating in early Tertiary time the angle was polarity (about a million years apart) close to 0°. (The angle we are mea are termed polarity epochs, and inter suring is the inclination of the earth's vals of shorter duration are termed axis to the orbital plane). polarity events. In short, the north Milankovitch took all of these and south poles remain in about their factors into account and constructed a present positions during the reversals, curve to show how the earth's climate but their polarity is reversed. The may have varied through geologic time. reversal seems to take place quite Presumably, if all three factors tended rapidly, over a period of only a few toward warming at the same time, then thousand years. Incidentally, the re the overall effect would be marked versals have been especially inter warming. Most geologists accept the esting to paleontologists because the validity of the Milankovitch curve, but magnetic field is eliminated during each there is considerable disagreement reversal, permitting increased cosmic about how important it is, what it radiation to reach earth. This in really means, and whether all the creased radiation may cause spurts of necessary factors are included and evolution. their relative importance. 16 Terms used Formation. Geologic for and is restricted to a on Figures 3 mations are the basic few ponds, lakes, and and others rock-stratigraphic units. streams--and therefore They consist of a body it can be said that we of rock of sediment are now experiencing a (limestone, shale, sand time of unconformity. stone, sand, etc.) that has a certain amount of Terms used Rancholebrean, Irving internal lithologic homo on Figures 4 tonian, etc. Southern geniety or other distinc and 5 California mammal zones. tive characteristic or The Rancholebrean zone characteristics. For is of upper Pleistocene example, the Pierre (Wisconsinan) age and Formation is mainly began about 80,000 to shale; the Red River 100,000 years ago. It is Formation is carbonate probably best known for rock. Generally, a for the tar pits that mation has lateral con trapped a great variety tinuity and covers a of animals, inclUding broad area. sabre-tooth tigers, dire wolves, etc. Other, Terms used Group. Groups, like older mammal zones on Figures 3 formations, are rock shown on figure 4, are and others stratigraphic units, but the Irvingtonian (also they consist of two or southern California), more formations that are Barstovian, Clarendo in some way related to nian, Hemphillian, and one another. For exam Blancan. Older mammal pie, the Coleharbor zones are not included Group consists of sev on the charts. eral similar formations, all of which are com Terms used Pearlette Ash, Bishop posed of glacier and on Figures Ash, Glacier Peak Ash, glacier-related deposits. 4, 5, and 6 Mt. Mazama Ash. Vol canic ash falls are Unconformity. A break important because they or gap in the geologic help geologists to pre record where a rock cisely correlate the ages unit is overlain by of geologic events over another that is not next wide areas. Thus, when in stratigraphic suc Mt. Mazama erupted cession is an uncon 6,600 years ago, the formity. Most important explosion formed a cra unconformities represent ter now flooded by an interruption in depo Crater Lake in Oregon sitional continuity that and the volcanic ash spans a considerable from the eruption fell length of time. Gener over much of North ally, the rocks below an America. The ash is of unconformity are con a specific chemical com siderably older than position and, wherever those above and they it is identified, it is have been eroded during possible to say with the time when deposition certainty that the rock was not taking place. or sediment it is found North Dakota is cur in is· 6,600 years old, rently undergoing thereby establishing erosion--deposition of regional age correla sediment is only minor tions. 17