Jefferson County Online A part of the IAGenWeb and USGenWeb Projects Geology ~ Lime Beds, Coal Basin, Cretaceous Period, Glacial Period, Drift Period, Bowlders, Economic Geology, and Origins of the Prairie

In the general history of the State which is given in this volume, will be found a somewhat elaborate description of the geology of Iowa, from a scientific standpoint. It remains for us to limit the circuit of our work in connection herewith to the actual boundaries of Jefferson County. We shall attempt to popularize a most interesting but not generally studied theme, and endeavor to explain, in simple form, what is too often rendered obscure to the uninitiated in scientific methods, by technical terms and expressions. Since those who wish to do so can turn to the general chapter and learn of the geologic structure of the State, let us now bring to a focus the more practical ideas relative to the subject of the recent or superficial formations of Jefferson County. This is designed to be only a short popular treatise, so as to interest every man and woman of good observation who shall peruse it, and to call their attention, at least, to the surface formation of the earth, so that in a few years there may be hundreds of observers of interesting geological facts where there is but one at the present time.

That geology commends itself to us as a truthful science will be very readily elucidated by a simple statement of a fact within the comprehension of all.

To illustrate: A certain kind of rocks are called Archæan or Laurentian. These are the most ancient rocks known to geologists; at one time they were supposed to be destitute of fossils. In all the systems of rocks, they occupy the lowest, and consequently the oldest, position; but in whatever part of the earth found, they are always recognizable by the geologist. So the Devonian rocks are distinguished by certain fossil fishes that are found in them, and in them alone. The Carboniferous rocks are known by certain fossil mollusks; the Cretaceous, by certain reptiles that occur in no other formation; and so every geological period has its characteristic fossils, by means of which the formation and its comparative age may always be accurately determined.

The geologist will always know the coal-bearing rocks from any other class; and this knowledge ought to be possessed by every one interested in explorations for coal.

The geologic history of Iowa is but a page in the general history of the continent of North America. This continent has been demonstrated to be the oldest portion of earth, notwithstanding the misnomer, "New World." It is only new in civilization. The geologist reads is the rocks evidences of age that are far more reliable than those which are placed on perishable scrolls by the pen of man. The oldest groups of rocks are not found in Iowa, but are visible in the Canadas. The first system, underlying all others, in this State, is the Azoic, seen only in a small section of the northeast portion of Iowa. Next come the Lower and Upper Silurian, the Devonian, the Carboniferous and the Cretaceous systems. Of the earlier formations we shall say nothing, as allusion to them necessitates a far more extended article than we desire to prepare.

The scope of this paper extends back only to the Carboniferous system, at the period known as the Subcarboniferous group. In plainer terms, this refers to the limestone which underlies the coal formations, and brings the subject at once to the visible formations in the valley. This section is rich in coal deposits, and a glace at the method of creation will be both interesting and instructive.

Limestones have mainly been formed in the bottom of the ocean; the older and purer kinds in the deep, still sea; the more recent and less pure in a shallow and disturbed sea. When the great limestone deposits were made in the Mississippi Valley, a deep salt ocean extended from the Alleghany to the Rocky Mountains, from the Gulf of Mexico to the Arctic Ocean. This was the age of mollusks (shell fish), and the sea bottom swarmed with them. Many of the rocks seem to have been wholly made up of conglomerate shells. In this age of the world there was no creature living with a spinal column or a brain; but corals, a low order of radiates, as crinoidea, several varieties of mollusks, crustaceans, called trilobites (somewhat corresponding to the river crawfish), and some lowly worms! These were the highest development of animal life when the earlier limestone rocks were being slowly formed.

This Silurian age was succeeded by the Devonian, characterized as the age of fishes, during which were deposited the Hamilton and Carboniferous limestones. Then came the Subcarboniferous period, during which were deposited the limestone beds. These were formed in a comparatively shallow sea, a fact proven by numerous ripple marks in the rocks, also by their sandy composition in some layers, and farther, by an occasional thin layer of clay intervening between the strata of rocks. these were uneasy times on the earth's crust, when it was given to upheavings and down-sinkings over large areas. Then it was that the whole northeastern and eastern part of the State was upraised.

was formed west and south throughout Iowa, reaching into Missouri and Kansas, the Indian Territory and perhaps Texas. Over this vast area there stretched a vast, dismal swamp.

On this vast marshy plain grew the rank vegetation that was in the future to be pressed into coal. It was a wilderness of moss and ferns and reeds, such as can be found nowhere on earth at the present time. Prof. Gunning, in speaking of it, says, "To the land forest of coniferas and cycads, and the marsh forest of scale trees and seal trees and reed trees and fern trees, add an undergrowth of low herbaceous ferns, and you have the picture of a primeval landscape. Blot from the face of nature every flowering weed and flowering tree, every grass, every fruit, every growth useful to man or beast; go, then to the Sunda Islands for the largest club moss, to the East Indies for the largest tree fern, to the damp glades of Caracas for the tallest reeds, to the Moluccas for their cycad, and to Australia for its pine, to the ponds and sluggish streams of America for their quillwort, and place them all side by side over a vast marsh and its sandy borders, and you will faintly reailze your picture of a primeval landscape. Dwarf the cycad and the pine, lift still higher the tapering column of the tree fern, multiply by two the bulk of the reed and by three the club moss, lift the quillwort from the water, and to its long, linear leaves add a fluted stem eighty feet high, and you would fully realize a carboniferous landscape -- realize it in all but its vast solitudes. Not a bird ever perched on spiky leaf or spreading fern of a coal forest. No flower had opened yet to spread fragrance on the air, and no throat had warbled a note of music. Such poor animal life as the carboniferous world then possessed left its imprint on wave-washed shore and in the hollow stems of fallen trees."

This was the beginning of the age of amphibians. Then lived the progenitors of the loathsome alligator and lizard. La Conte says: "The climate of the coal period was characterized by a greater warmth, humidity, uniformity and a more highly carbonated condition of the atmosphere than now obtains." We may, therefore, picture to ourselves the climate of this period as warm, moist, stagnant and stifling from the abundance of carbonic-acid gas.

Such conditions were extremely favorable to vegetable life, but not to the higher forms of animal life. Neither man nor monkey nor milk-giving animal of any kind, lived for many cycles of time after the Subcarboniferous period; but that vegetation grew rank, scientific facts corroborate; thus, Prof. gunning says: "It takes between five and eight feet of vegetable debris to form one foot of coal. A Pittsburgh seam is ten feet thick, while one in Nova Scotia is thirty-five feet in depth. the Pittsburgh seam represents a vegetable deposit of from fifty to a hundred feet in depth, and the one in Nova Scotia between an hundred and seventy-five and three hundred and fifty feet in thickness. A four-foot seam in Wapello County would represent from twenty to forty feet of vegetable debris.

During the growth and decay of this vegetable matter, the surface of the earth did not sink; but this quiescent period was followed by one of submergence. "The surface, loaded with the growth of quiet centuries, was carried down beneath the sea, where it was swept by waves and overspread by sands and mud." It was in nature's great hydraulic press, where it remained until another upheaval again threw it to the surface, and another long area of verdure succeeded the one of submergence.

Thus, emergence and submergence succeeded each other as many times as the coal-seams and the shale, slate or sandstone alternate -- in some parts of Iowa, three times, in Nova Scotia about forty times! Who can compute the centuries here recorded!

The coal-fields of Iowa are extensive. A line drawn on the map of the State as follows will about define them: Commencing at the southeast corner of Van Buren County, running to the northeast corner of Jefferson, by a waving line slightly eastward through Lee and Henry Counties; thence a few miles northward from Jefferson and northwestward, keeping six or eight miles north of Skunk River, until the southern boundary of Marshall County is reached a little west of the center; thence three or four miles northeast from Eldora, in Hardin County; thence westward to a point a little north of Webster City, in Hamilton County, and thence westward to a point a little north of Fort Dodge, in Webster County.

The coal-field in Iowa belongs to the true carboniferous system, and is, moreover, the outfield of the vast coal-basin which partly covers this State, Illinois, Indiana, Ohio and Pennsylvania. It is only in the Alleghanies that subterranean action has converted any part of the coal into anthracite. Everywhere else in the immense basin it is strictly bituminous, varying, however, from the article as first prepared by the economic forces of Nature from the block coal of Indiana to the cannel coal found in certain parts of Iowa.

It appears from the researches of Liebig and other eminent chemists, that when wood and other vegetable matter are buried in the earth, exposed to moisture and partially or entirely excluded from air, they decompose slowly and evolve carbonic acid gas, thus parting with a portion of their original oxygen. By this means they become gradually converted into lignite, or wood coal, which contains a larger proportion of hydrogen than wood does. A continuance of decomposition changes this lignite into common or bituminous coal, chiefly by the discharge of carbureted hydrogen, or the gas by which we illuminate our streets and houses. According to Bischoff, the inflammable gases which are always escaping from mineral coal, and are so often the cause of fatal accidents in mines, always contain carbonic acid, carbureted hydrogen, nitrogen, and olefiant gas. The disengagement of all these gradually transforms ordinary or bituminous coal into anthracite, to which the various names of glance coal, cota, hard coal, culm and many others have been given.

In explaining the cause of the freedom of coal from impurities of almost every description, Sir Charles Lyell gives a paragraph which is interesting in this connection. He says: "The purity of coal itself, or the absence in it of earthy particles and sand, throughout areas of vast extent, is a fact which appears to be very difficult to explain when we attribute each coal-seam to a vegetable growth in swamps. It has been asked how, during river inundations capable of sweeping away the leaves of ferns and the stems and roots of trees, could the waters fail to transport some fine mud into swamps? One generation of tall trees after another grew in mud, and their leaves and prostrate trunks formed layers of vegetable matter which afterward covered with mud and turned to shale; but the coal itself, or altered vegetable matter, remained all the while unsoiled with earthy matter. This enigma, however perplexing at first sight, may, I think, be solved by attending to what is now taking place in deltas.

"The dense growth of reeds and herbage which encompasses the margin of forest-covered swamps in the valley and delta of the Mississippi, is such that the fluviatile waters, in passing through them, are filtered and made to clear themselves entirely before they reach the areas in which vegetable matter may accumumate for centuries, forming coal, if the climate is favorable. There is no possibility of the least intermixture of earthy matter in such cases. Thus, in the large submerged track called "Sunk Country," near New Madrid, forming part of the western side of the valley of the Mississippi, erect trees have been standing ever since the year 1811-12, killed by the great earthquake of that date; lacustrine and swamp plants have been growing there in the shallows, and several rivers have annually inunudated their whole space, and yet have been unable to carry in any sediment within the outer boundaries of the morass, so dense is the marginal belt of reeds and brushwood. It may be affirmed that generally, in the cypress swamps of the Mississippi, no sediment mingles with the vegetable matter accumulated there from the decay of trees and semi-aquatic plants. As a singular proof of this fact, I may mention that whenever any part of the swamps in Louisiana is dried up, during an unusually hot season, and the wood is set on fire, pits are burned into the ground many feet deep, or as far down as the fire can descend without meeting with water, and it is then found that scarcely any residuum or earthy matter is left. At the bottom of these cypress swamps a bed of clay is found, with roots of the tall cypress, just as the under clays of the coal are filled with stigmaria."

The next formation above the coal was the cretaceous, or chalk. This formation is not seen in this region, being encountered only in the west and northwest portions of the State. If any ever existed here, it was carried away during the glacial period, which is hereafter explained. The absence of chalk brings us to speak next of the

That the surface of Iowa, and, in fact, the whole of North America north of the thirty-eighth parallel, is covered by a material known as drift, has become a popular opinion. Strewed all over the country, on the hills and in the valleys and on the level prairies, covering up the native rocks to a depth of from twenty to three hundred feet, is found this peculiar deposit. The well-diggers and the colliers, in their excavations, encounter it, and the quarryman has to strip it from the surface of this rock bed. It is not all alike; first there are a few feet of surface soil, created by recent vegetable deposit; then a variable depth of clay, or clay and sand intimately blended; then water-worn gravel and sand, and then blue clay, resting upon the country rock.

Scattered over the continent are frequently seen "lost rocks," or bowlders, of various sizes and of different varieties, some of granite, others of gneiss or trap, and occasionally some of limestone. These bowlders are also frequently found in excavating the earth.

The blue clay which lies upon the country rocks, or the original formation, is the oldest of the drift deposits. It consists of a heterogeneous mixture of dark blue clay, sand, gravel, pebbles and irregular-shaped stones and bowlders, of various kinds and sizes, unassorted and unstratified, and therefore could not have been deposited in water. Sometimes an occasional piece of stone-coal and fragments of wood are found in it. This blue clay is bowlder or glacier clay. From whence it came and how formed is one of the most interesting subjects that scientific minds have investigated. This history of glacial phenomena is the history of the deposition of the blue clay formation.

Too much credit cannot be given to the late lamented Prof. Agassiz and Principal Forbes for their discovery of the laws regulating glacial action. These eminent savants built a hut on a living glacier, in Switzerland, and studied it in all its relations to the past history of the globe.

Prof. Gunning says: "The area of Greenland is nearly eight hundred thousand square miles; and all this, save a narrow strip which faces an ice-choked sea, on the west, is a lifeless solitude of snow and ice. The snow overtops the hills and levels up all the valleys, so that, as far as the eye can reach, there is nothing but one vast, dreary, level expanse of white. Over all broods the silence of death. Life, there is none. Motion, there seems to be none -- none save of the wind, which sweeps now and then, in the wrath of a polar storm, from the sea over the 'ice-sea,' and rolls its cap of snow into great hillows, and dashes it up into clouds of spray. But motion there is; activities we shall see there are, on a scale of grandeur commensurate with the vast desolation itself."

Let the mind go back in the history of our earth, one hundred thousand years, when, Prof. Croll, from mathematical deductions, infers the existence of a snow cap, covering the whole of North America and Europe, from the thirty-eighth parallel to the north pole; then, in imagination, see the larger portion of North America, as you see Greenland now, covered with an "ice-mantle" 3,000 to 6,000 feet thick. A glacier is a frozen river, having motion as a stream of water has, but bound in gigantic bands by the cold atmosphere. Conceive, if you please, a moving block of iron, thousands of tons in weight, dragged over a plowed field. The track of this monster is marked by a level bed of compressed, pulverized earth. Transfer your imagination to a mass of ice covering the entire northern hemisphere, or at least to the thirty-eighth parallel (at which point the equatorial heat began to assert itself on the ice-walls, and decompose them, carrying the debris of the glacier, in solution, southward), moving half a foot or more a day, because of the hydraulic pressure from behind and within -- the streams which flowed into it -- and you can then have some faint idea of the incalculable force of a glacier, and the action of the ice-mass on the plastic earth.

The dynamic power of such a continental mass of ice is inconceivable. It is fit to be called one of the giant mills of the gods, which are represented "to grind slowly, but exceeding fine." It was a monstrous ice-plane, shaving off the rugged crags of mountains, leveling up valleys and filling up ancient river-beds. Its under surface was thickly set with rock-bowlders, which, with its ponderous weight, ground the underlying rocks to powder. This pulverized rock was washed from beneath the glacier by the overflowing waters which constantly gushed forth, and settled on far-off plains as alluvial sand and clay. The motion of the glacier was slow, perhaps six inches in twenty-four hours. This was the giant mill that ground out the blue clay -- the glacier clay -- that overlies the native formations of the entire country. It doubtless owes its dark blue color to the Laurentian and trap rocks of Canada. Well-diggers are familiar with it and it is nearly always the same in color and composition. Geologists are now unanimous in the opinion that during the glacial epoch the whole northern portion of the continent was elevated one thousand to two thousand feet above the present level.

Le Conte says: "The polar ice-cap had advanced southward to 40° latitude, with still further southward projections, favored by local conditions, and an Arctic rigor of climate prevailed over the United States, even to the shores of the Gulf. At the end of this epoch an opposite or downward movement of land surface over the same region commenced and continued until a depression of five hundred or one thousand feet below the present level was attained.

Le Conte says: "This ice sheet moved, with slow, glacier motion, southeastward, southward and southwestward, over New England, New York, Ohio, Illinois, Iowa, etc., regardless of smaller valleys, glaciating the whole surface, and gouging out lakes in its course. Northward, the ice-sheet probably extended to the pole; it was an extension of the polar ice-cap."

It is not within the province of this sketch to go into details and give the problematic causes of this glacier period. The causes were mainly astronomical. Mr. Croll has calculated the form of the earth's orbit a million years back and a million years forward. The probable time of the last glacial period was 100,000 years back; then the eccentricity of the earth's orbit was very great, and the earth in aphelion (or when most distant form the sun, being about thirteen millions of miles further than in summer) in midwinter; then the winters were about thirty days longer than now. In summer, the earth would be correspondingly nearer the sun, and would receive an excess of heat, thus giving the earth in the northern hemisphere short, hot summers and long, cold winters.

The subsidence referred to above forms the beginning of

Now let us see how the drift was deposited on the bowlder clay. When the continental depression took place, a large portion of the Mississippi Valley was submerged. Le Conte says: "It was a time of inland seas. Another result, or at least a concomitant, was a moderation of the climate, a melting of the glaciers, and a retreat of the margin of the ice-cap northward. If was (sic), therefore, a time of flooded lakes and rivers. Lastly, over these inland seas and great lakes, loosened masses of ice floated in the form of icebergs. It was, therefore, a time of iceberg action."

For a time the ideas upon the subject of glacial and iceberg action were confused, until Prof. Agassiz practically demonstrated the difference, on the glacier in Switzerland. The iceberg period followed that of the glacier. The depression of the continent, from 1,000 to 2,000 feet, created a sea-bed. This was filled by the melting of the glacier. Meanwhile, the water supply on the glacier continued, but the moderated climate prevented the formation of the ice-cap. As a result, the hydraulic pressure from behind forced the glacier, or frozen stream, into the sea. the buoyancy of the water counteracted on the specific gravity of the glacier, and, when the ice had projected beyond a point at which it could resist the upward pressure of the sea-water, great masses of it were broken off. These masses floated away, and are known as icebergs.

The glacier was frozen to the bottom of its river-bed, congealing in its embrace rocks, gravel, sand and whatever substances lay thereon. These substances were held firmly during the progress of the iceberg, after its liberation from the parent glacier, until it had floated into warmer waters. Then began a gradual dropping of the freight of the berg, until finally the ice itself disappeared in the mild waters of a tropic ocean.

The opinion prevails among geologists that the glacier motion was from the east of north, but that the Champlain flow was from the northwest. Corroborating this hypothesis is the marked difference in color of the bowlder clay and the Upper Drift deposit. If the glacier motion was from the north, or east of north, it did not produce the beds of our present rivers. Glaciation, or the process of leveling the earth's surface of the country, leaving it a vast undulating plain of dark blue mud, a heterogeneous mass of clay, sand, gravel and bowlders. The old river courses and valleys were completely obliterated. That the great beds of alluvium which cover up the blue clay were deposited in water, is clearly proven by its stratification, which can be observed in almost any excavation where a hill or bluff has been cut through in constructing railroads or mills, or where brick clay has been procured.

But let us see how the Champlain or Drift period was produced.

A continental subsidence came on and large inland lakes were formed. The climate became modified; the glaciers melted more rapidly; vast icebergs broke loose from the mountain-like glaciers and floated over the land, carrying rocks and clay and debris with them. and as they melted, strewed them over the surface, sometimes grounding and excavating basins for future lakes and ponds. Thus, year after year and age after age, did the muddy waters and freighted icebergs flow over the country, the former depositing our present alluvial drift, the latter dropping here and there the bowlders and debris that we now find scattered over the country. No erosion or wearing away, save from a stranded iceberg, occurred at that time, but it was a period of filling in, a period of distribution over the submerged land, of powdered rocks, sand and clay, and an occasional bowlder. But when the continent emerged from the abyss, and the waters flowed off, and the higher undulations of the land appeared, then the erosive action of winds and waves and storms and currents took place. The waters, as they flowed toward the sea and Gulf, produced their inevitable channels.

There was much of the drift carried into the streams and borne away in the floods to the sea. Then was the stranded bowlder, by wind and wave, stripped of its soft, alluvial bed, left high and dry on the surface of the hereafter prairie. Then there were the gravelly knolls that are found in some parts of the State robbed of every fine sediment, and the gravel and stones left to tell the story of the floods. Then were the great valleys washed out; then did the annual washouts along the water-courses -- rapidly at first, but more slowly in after ages -- eat away at the drift accumumations and form the hills. The hilly districts generally lie contiguous to the streams. Back from these water courses the land is usually undulating prairie, showing but little erosion.

The country contiguous to the Des Moines River and its tributaries bears, in many localities, unmistakable evidences of the action of the retiring waters of the Champlain period. As geology has written its history in the rocks, so the latest action of the waters has left its legible records in the drifts -- it made tracks, and by its tracks we can see where it was and what it did.

When two currents of water flow together, charged with sediment, where the currents meet there will occur an eddy, the eddy-water will throw down its load of floating mud and build up a bar. In the valley of every creek in this locality, may be found many of those silted-up banks and promontories, the deposits of the waters during the later Champlain period.

If our readers will but notice the action of any swollen creek, they will at once perceive how the prairie streams have silted or thrown up the hillocks so frequently met with. Notice the little brook that meets the larger creek yonder. At the mouth of the brook is a firmer bit of ground in the slough, upon which the horseman, at an early day, safely crossed the miry ford. That firm ground was formed by the heavy sediment of the brook. The two streams produced an eddy on meeting, and the waters were delayed an instant. Some of the sand brought down stream sank during this pause, and a hillock in embryo was made.

Years from this time, the course of that stream will be changed because of an impeding elevation of the land, and that elevated land will be cultivated, with rich returns. So the surface of the prairies was formed into irregular hills and dales.