History of Linn County Iowa Part 4

The lowest beds of the Silurian belong to the _Hopkinton stage_, and are exposed along the Buffalo. At Hill's mill and at Nugent's quarries some layers are crowded with a characteristic fossil--a plump bivalve sh.e.l.l as large as a walnut, which goes by the name of _Pentamerus Oblongus_. The _Gower_ stage of the Silurian rests upon the Hopkinton and embraces two types of rocks distinct in their appearance and uses.

The _LeClaire phase_ of the Gower is a hard, brittle, crystalline, magnesian limestone, or dolomite. Normally blue-gray in color, it is often oxidised to buff. It is well exposed at Viola and on the Cedar river from the Cedar County line to a mile or so beyond the Upper Palisades, southwest of Bertram. The LeClaire forms mounds in places reaching fifty and even eighty or ninety feet in height in which little semblance of bedding structures are to be seen. Here and there the rock is conglomeratic, consisting of rounded ma.s.ses of the rock cemented by a less resistent matrix. The cavernous recess in the rock wall of the Palisades, misnamed the Blowout, is due to the solution of the weaker matrix and the dislodgement of the rounded ma.s.ses. The rock may consist also of angular broken blue-gray fragments in the matrix of a buff and friable limestone sand. Again, the mounds, at least in part, may be made up of ma.s.sive limestone with little trace of structure of any sort. On the sides of the mounds and merging into the conglomeratic or other structures the rock of the LeClaire often is stratified and the layers dip outward at angles surprisingly high. In places these tilted layers may show sharp folds. The rock of all structures is fossiliferous. Even the broken fragments of breccia are porous with moulds of minute fossils which have been removed by solution. The ma.s.sive rock is largely made up in places of stems of crinoids--stone lilies which grew in the greatest profusion in these quiet waters--and the tilted layers may be made of casts and moulds of unbroken sh.e.l.ls of little bivalves. Occasionally the saucer shaped tail and head-s.h.i.+elds of a characteristic trilobite are found piled together and unbroken.

Coral are very common in this ancient reef rock, a form resembling honeycomb being especially noticeable. And as one floats down stream at the base of the cliffs he can hardly fail to notice large tapering segmented sh.e.l.ls, either straight or slightly curved, representatives of the cephalopod mollusks.

The picturesque rock walls of the Palisades, which rise perpendicular for as much as ninety feet from the water's edge, are due primarily to the great resistance of the LeClaire rock, due to its chemical composition--for dolomite weathers far less rapidly than a non-magnesian limestone--and to the fewness of those planes of weakness called joint-planes. The joints of the LeClaire are distant and vertical. The stone breaks down, therefore, in immense blocks where undercut by the river which leave for ages the scarp behind them as a vertical wall.

Because of its qualities the LeClaire is one of the best lime rocks in the country. The impurities of the clay, the iron and silica which it contains, may run as low as one-third of one per cent. The large per cent of carbonate of magnesia present makes it a cool lime, slow to slack and slow to set, and it is to such limes that architects, masons, and plasterers now invariably give preference over the so-called hot limes burned from non-magnesian limestones. The hardness and durability of mortars made from the LeClaire rock limes approaches that of cement, and after thirty-five or forty years of weathering, joints in mason work seem almost as fresh as when first struck.

The extreme hardness of the rock and the slowness with which it weathers make it specially valuable for crus.h.i.+ng for macadam and ballast.

The _Anamosa phase_ of the Gower limestone is typically exposed in the large quarries at Anamosa and Stone City, Mount Vernon and Waubeek. It is a light buff or yellow limestone, with constant, parallel, and horizontal or gently inclined laminated layers. The limestone is soft to work but hardens on exposure. The saw encounters no obdurate materials and the chisel finds the fracture even and regular. Bedding planes are so even and smooth as to be at once ready for the mortar with little or no dressing. Much of the stone can be split horizontally to any desired thickness, while the distant joints permit the quarrying of blocks beyond the facilities of transportation or any possible use.

Many layers are so h.o.m.ogenous that they can be wrought into fine carvings.

As a dolomite the stone is far more resistant than a purer limestone.

In the Mount Vernon cemetery tombstones of this material, whose dates run back to the forties and early fifties, have been so little affected by superficial decay that the tool marks are almost as fresh as when the chisel left them; while marbles of half their age have broken down into ruin.

The Silurian rocks of the county measure about 300 feet in thickness.

They are confined pretty closely to the towns.h.i.+ps of the eastern tier, but extend beyond their limits up the valleys of the Cedar and Wapsipinicon.

THE BERTRAM LIMESTONE

As the Silurian limestones sink below the surface because of the westward dip, they are succeeded by a bed of rock, named from its outcrop at Bertram, and found along Big Creek as far north as Paralta and Springville. This is a heavily bedded gray rock which weathers almost white. At a number of places along Big Creek it forms picturesque cliffs, and hillsides covered with huge boulders of disintegration. At one point it is seen to overlie the Anamosa beds of the Silurian, and several exposures are known where it is succeeded by the Otis limestones of the Devonian. But as it contains no fossils, so far as is now known, it can not be said to which of the two ages it belongs.

THE OTIS LIMESTONES

The lower beds of the Otis, as exposed at the base of the Otis quarries, along the Cedar south of Cedar Rapids, at Springville, and at Coggon, consist of soft magnesian limestones, fossiliferous with many moulds of small bivalve sh.e.l.ls of Devonian age. These pa.s.s upward into drab non-magnesian limestones carrying the princ.i.p.al fossil of the magnesian beds in considerable numbers. The upper limestones of the Otis differ within rather wide limits. The most common type is seen at the base of the high cliff at Kenwood on the right bank of Indian Creek--a hard, brittle ringing and thinly laminated limestone. Often it has been subjected to strains under which it has broken, and has been re-cemented with little displacement of the parts. Occasionally it is brown, and highly crystalline.

THE INDEPENDENCE

At the Kenwood cliff the eight feet of the Otis at the base is succeeded by thirty feet of buff shale and clayey limestones--a formation known as the _Independence_ from its discovery in a shaft sunk at that city. The Independence is exposed at many points near Cedar Rapids both on Indian Creek and on the Cedar. On the Wapsipinicon it is well seen at Cedar Bluff (sec. 24 Spring Grove Tp.), at the "Wolf's Den" a mile up valley, and again in the railway cut north of Coggon. In the long cut of the Chicago, Milwaukee & St. Paul Railway west of Linn Junction the Independence is seen in one place as a blue clay shale carrying a number of fossils characteristic of the shaft at Independence, but elsewhere the formation is unfossiliferous in the natural exposures so far studied.

Wherever found the Independence contains nodules of silica, which may reach a foot in diameter, and often angular fragments of the same material which may be as fine as sand. The formation is marked by irregularities of deposition, channel cutting by drift currents, lenses of calcerous material, and rapid lateral change in the form and const.i.tuents of the rock. All of these characteristics point to the deposition of this formation in a shallow sea near sh.o.r.e.

Indeed, some of the beds were apparently laid in marshes such as are now found along low ocean sh.o.r.es. Thin seams of coal formed in the Independence were once peaty deposits preserved by the presence of water from the decay which returns dead vegetable matter to the air. In 1871 such a seam of coal, not exceeding an inch in thickness, was found at a depth of ninety feet in a well on the farm of Mrs. C. Hemphill, near LaFayette. Pieces of the coal were taken to Cedar Rapids and Marion. A mining company was formed, and without seeking for any expert advice from geologist or mining engineer, and without any tests of the extent and thickness of the seam, a shaft was sunk after the precious fuel. Water was encountered in such quant.i.ties that expensive pumping machinery was used, and in all several thousand dollars were wasted in a search which any competent geologist could have told was foredoomed to failure.

THE DAVENPORT LIMESTONES

The sea over eastern Iowa deepened after the deposition of the Independence, for there was now deposited upon its floor limestones in place of shales. The lowest of these, known as the _lower Davenport_ beds, are hard, compact, and of finest grain, and so far as known are unfossiliferous. The _upper Davenport_ is a tough, gray, semi-crystalline limestone which contains an a.s.semblage of fossils of many species. Highest of these are the first vertebrates to appear in Iowa so far as our records go. Fishes which swam over our area left to be imbedded in the limestones their hard enameled teeth and fin spines.

The most common of the Devonian fishes was a small shark.

In several other counties the lower and the upper Davenport limestones retain the att.i.tude of their deposition. But everywhere in Linn county they have been broken into bits and re-cemented, forming breccia. These brittle rocks could hardly give way to such immense stresses without causing sharp and violent vibrations to run through the crust of the earth, and we may therefore list great earthquakes as a part of the history of our area in Devonian times.

The best exposure of the breccia beds is that of the cut of the Chicago, Milwaukee and St. Paul Railway west of Linn Junction. The brittle lower Davenport has here been broken and rebroken into a ma.s.s of small sharp-edged fragments, while the tough heavily bedded upper Davenport ledges have been fractured to large blocks, which sliding on each other have smoothed and grooved their sides. The breccia beds may be seen in the upper eleven feet of the Kenwood cliff, at Troy Mills, and in the cliffs along the Wapsipinicon valley as far down as near to Central City.

THE CEDAR VALLEY LIMESTONES

The Otis, Independence, and Davenport limestones form a group called the Wapsipinicon, from its outcrop along the river of that name in Linn county. The remaining limestones of the Devonian are grouped together under the name of the Cedar Valley. These consist of limestones of various types, sometimes crowded with fossils, and sometimes dest.i.tute of any trace of ancient life. They occupy the western towns.h.i.+ps of the county.

THE CARBONIFEROUS

At the close of the Cedar Valley stage the sea retreated westward from our area, and Linn county became dry land. For long ages its rocks were covered with rich soils supporting a luxuriant vegetation, probably tropical in its aspect. We know that running water channelled this ancient land, for when at last in _Pennsylvanian_ (Coal measure) time the land sunk slowly beneath the sea, there were deposited in such channels clays and sandstones, which perhaps are only remnants of wide sheets of similar deposits now removed by denudation. A mile and a half south of Marion (southeast quarter of section 12, Rapids towns.h.i.+p) a well twenty-three feet deep penetrated a bed of dark shale which carried leaf impressions of a number of ferns characteristic of the undergrowth of the Carboniferous forests. A third of a mile southeast of Lisbon, and again about two miles south of the same village, at Bertram at the east end of the railway bridge, and on the old county road between Cedar Rapids and Marion, are exposures of sandstone which in some instances contain fragments of the logs drifted from perhaps distant uplands and water-logged and sunk in these ancient sand beds.

The Bertram outlier contains many rolled coral fragments and worn bits of sh.e.l.ls of the Devonian, included in Carboniferous deposits, much as the same fossils may now be found in the river deposits of the present age in the sand bars of the Cedar.

MESOZOIC AND TERTIARY

For a succession of geologic ages our county, in common with eastern Iowa, seems to have remained dry land, for no deposits of the sea are found upon it. On both sides of the continent mountain ranges of Alpine height were uplifted, and during the immeasurable years worn down, grain by grain, to flat and featureless plains. But no deformations are recorded in our county history and the lands seems to have remained so low that little erosion was possible. We are permitted to conceive that over our savannas in Mesozoic times there roamed monstrous reptiles of strange shapes, such as are known to have existed in adjacent states.

In the later ages of this era it is not impossible that during the great submergence which brought the Cretaceous sea over the Great Plains from the Arctic to the Gulf, including western Iowa, our area also may have been inundated and huge swimming reptiles such as are found in the deposits of Kansas and Nebraska may have disported themselves where now our rich farm lands lie open to the sun, while in the air featherless cold-blooded creatures larger than any bird winged their way on leathery pinions.

[Ill.u.s.tration: THE ASTOR HOUSE Erected by John Young in 1839, Looking South The Second House in Cedar Rapids COURTESY CARROLL'S HISTORY]

During the millions of years which are included in the Tertiary ages Linn county was undoubtedly dry land. On our gra.s.s lands pastured a succession of strange and uncouth mammals evolving into higher and higher forms. Among these denizens of the county were probably herds of pig-like creatures, three toed horses little bigger than foxes, and ancestral monkeys swarming in the trees, for such are known to have existed in other states. But these chapters in the history of the county can not be written from any local records.

THE GLACIAL EPOCH

The warm climate of Tertiary times changed slowly to one of arctic cold. The winters lengthened and the summers becoming ever cooler and yet cooler failed at last to melt the winter snows. Vast sheets of glacial ice, such as that which shrouds Greenland today, covered much of the continent. The geologic panorama thus presents our area as buried beneath one after another of slow-moving glaciers hundreds of feet thick. The proofs of their existence are found in almost every cutting which goes below the soil. Any quarry will show the rock deeply rotted and pitted by long preglacial decay. Here and there upon its surface will be found remnants of the deep red residual clays, the subsoils of preglacial times. Upon these clays formed from the decaying rock rest stony clays in which clay, sand, and stones faceted as only glacier ice can facet, are mingled pell-mell together, as only glacier ice can mingle. Occasionally is found the unmistakable track of the glacier left on the underlying rock sc.r.a.ped smooth and marked with parallel scorings, as at the north end of the cut of the Chicago, Milwaukee & St. Paul Railway at Linn Junction.

The glaciers also brought from ledges of granite and other crystalline rocks in Minnesota, Wisconsin, and Canada the boulders which form a conspicuous feature in some of our prairie landscapes. These, the "first settlers," traveled to their destinations far more leisurely than any ox carts of the immigrant pioneers; for the glaciers can not have moved faster at most than fifty feet a day, and probably at less than a tenth that rate, judging by the rates of motion of present glaciers.

The ice sheets of the glacial epoch plastered the county thick with the stony clays which they dragged along in their basal layers. The thickness of these glacial deposits probably averages from fifty to one hundred feet. Old valleys cut in rock by Tertiary rivers were buried wholly from view, as, for example, one extending north from Prairieburg; and the farmer now plows his corn in fields which lie two and three hundred feet above the channels of ancient rivers. In places the old valleys were left to be re-occupied by the rivers. Such are the reaches of wide valley of the Cedar south of Center Point. In other places the rivers were diverted wholly from their ancient beds and made to flow in new channels which they have not yet had time to widen and deepen to their ancient measures. Such are the narrow rock bound valleys of the Wapsipinicon south of Troy Mills and of the Cedar at the Palisades.

On the final retreat of the glaciers waters from the melting ice swept over the county, leaving deposits of sand on the lower lands and in the valleys. Since the glacial epoch the rivers have cut their beds a score of feet and more below the deposits of glacial floods and in many places, as near the Ivanhoe bridge, remnants of these ancient flood plains are left as terraces or "benches" or "second bottoms." At Bertram the sands deposited by glacial waters near the mouth of Big Creek stand about fifty feet above the level of the river.

THE LOESS

A large part of the county is covered with a deposit of fine yellow silt called loess. Dry, it crumbles into powder at a finger touch; wet, it is somewhat plastic and can be moulded into brick and tile. On the hill and uplands the loess is thickly spread, adding in places at least forty feet to their elevation. Over the lowlands it is thin or absent.

This yellow earth has been and is to be of greater value than mines of yellow gold. It is of inexhaustible fertility. It contains abundant mineral plant foods, partly const.i.tuent, and partly brought up into it by ground water; and these foods are so finely pulverized as to be of readiest solution and absorption by the roots. In wet weather the loess mantle absorbs the rainfall like a sponge; in months of desert drouth, like those of the summer of 1910, it returns the water to the surface, like a wick, to preserve the crops from failure.

A disadvantage of the loess lies in the readiness with which it washes.

The forest which once covered nearly all the uplands protected the soil from wash by means of its mattress of roots and the thick prairie sod was equally efficient where hill slopes were gra.s.sed over. But where forests have been thoughtlessly cut down, and steep slopes turned to plow land, it is but a few years until the brown top-soil is all washed away and the fields in spring when freshly plowed are as yellow as a deep cut in road or brick yard. The foot path in the pasture or the furrow of the plow becomes a gully in a single heavy rain, and unless checked soon becomes a gulch scores of feet in width.

By accenting the height of the ridges the loess also adds to the scenery of the county. Our area lies in a part of east central Iowa where the stony clays deposited by ancient glaciers acc.u.mulated in long ridges and belts of upland rising many feet above the intervening undulating plains. Because of the alternation of ridge and lowland no part of the state except the valley of the Upper Mississippi has so beautiful and wide and varied prospects. Over more or less of their course the rivers of the county have cut their channels lengthwise in the ridges, thus giving rise to the bold scenery of the Wapsipinicon above Central City, and of the Cedar near Mount Vernon. Some of these picturesque reaches of river and cliff and forest slope should surely be converted into county parks in the near future and preserved for the gratification of all coming generations. Unless this is done we may expect that the forests will be cut down and the hill slopes gashed with countless gullies; while the lichened rocks of the river cliffs fringed with fern and tamarisk will give place to unsightly quarries.

While Linn county was sheeted with glacier ice, no life of any sort was possible within its limits. But during the long interglacial epochs which intervened between the ice invasions, forests grew and animals now extinct roamed over our hills and plains. Among these early inhabitants may be mentioned extinct horses and the giant proboscidians, the mammoth, and the mastodon. These returned to the area after the final retreat of the ice and their remains are found in the peat bogs and river gravels. In the earliest of the interglacial epochs it is quite probable that some of the gigantic groundsloths of South America made their home here, since they are known to have done so in the western counties of the state. No traces of man have been found in the glacial deposits of Iowa, nor have any indubitable evidences of his presence in glacial times been found in North America.

Sometime, we know not when, roving tribes of Indians set foot within our area, and geology gives place to archeology. And when the white man appeared, inductive history ends and there begins the history of tradition and written records.

CHAPTER VI