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 shells of Devonian age. These pass upward into drab non-magnesian limestones carrying the principal 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 constituents of the rock. All of these characteristics point to the deposition of this formation in a shallow sea near shore.
Indeed, some of the beds were apparently laid in marshes such as are now found along low ocean shores. 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 quantities 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 assemblage 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 attitude 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 mass 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.