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 shell 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 masses 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 masses. 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 massive 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 massive 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 shells of little bivalves. Occasionally the saucer shaped tail and head-shields 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 shells, 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 crushing 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 homogenous 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 townships 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.