Part 4 (1/2)
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.