A FIRST SETTLER, NEAR MT. VERNON
PALISADES
The most recent of the formations which are pierced by the drill, but which do not come to the surface within the limits of the county, is the Maquoketa shale, reached in the eastern townships at a depth of somewhat more than 300 feet. This impervious bed of altered clay stops the descent of ground water, which thus is stored in large quantities in the overlying limestones and supplies some of the important wells of the county such as that of the town of Mount Vernon. At the time when these sea clays were laid, eastern Iowa was under sea, but so near was the low lying land to the north and east that vast quantities of mud were brought in by its rivers forming deposits nearly 300 feet in thickness.
THE SILURIAN
With the lapse of ages physical conditions changed and Linn County was covered with a warm shallow coral sea in which were laid the massive limestones which now form the country rock in the eastern tier of townships. In some of the quarries one may see the ripple marks into which these coral sands were heaped by the pulse of the waves, and one may pick out of the rocks casts and moulds of ancient sea shells, corals, and trilobites, which formed the highest forms of life then tenanting the Iowa seas.