Before the glaciers wrote their chapter in the long geological history of what is now Indiana, a great drama had been enacted and recorded in the rocks. All the underlying rock strata found in Indiana are sedimentary rock, formed -- in the course of thousands of centuries -- along the margins and at the bottom of the seas. At some time the preCambrian seas covered the Indiana area, and during this earliest period the first sedimentary rocks were deposited upon the original earth crust. Rocks formed during this time are exposed at no point in the State today; but their presence has been discovered, in digging deep wells, beneath all the strata of later periods.
Except for this basic layer, all the rock strata in Indiana were deposited during the Palaeozoic era, an incalculably long period within which most of the lower orders of plant and animal life developed. There were, in order of time, six great subdivisions within the Palaeozoic: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous (including the Mississippian and Pennsylvanian), and Permian periods. Rocks of these periods are identified by their plant and animal fossil remains.
During the Cambrian, Ordovician, and most of the Silurian periods, Indiana was submerged beneath the seas. In the later Silurian a mighty upheaval began; eventually most of the continent was uplifted and the great interior seas slowly receded. This was not a violent or sudden process; the earth rose only an inch, perhaps, in a century or more.
In the Indiana region the first and sharpest uplift was the formation of what is now called the Cincinnati Arch. Pressure from the earth's crust slowly forced upward the layers of rock formed in the preceding periods. These layers were pressed into a kind of long ridge, from which they sloped on either side to form flanks. As it slowly reared itself, the arch divided the sea of the Indiana region into two basins -a relatively small one to the north and a larger southwestern sea. In Indiana this ridge (or rather, the 'stubs' of its flanks, for the ridge was worn away by thousands of centuries of weathering) extends northwest from Cincinnati to Richmond, thence to Kokomo, Logansport, and Chicago.
Most of the Cincinnati Arch was formed in the late Silurian and Devonian periods; but the slow process of continental upheaval continued throughout the Palaeozoic era. In the Mississippian and Pennsylvanian epochs of the Carboniferous period, Indiana was steadily elevated; at the close of the Mississippian the whole region was above sea level. During the Pennsylvanian, a period of millions of years, Indiana was probably a rank, lush swamp -- populated by amphibious creatures, and covered with fern-like plants growing in vast luxuriance.
In the Permian period the swamps dried and the climate became cooler. Seas never invaded the Indiana region again.
With this brief outline in mind, the geological formations of Indiana are easily understood. In the southeastern corner of the State the surface rocks, immediately under the topsoil, are of Ordovician age. Then in order, toward the west, appear belts of Silurian, Devonian, Mississippian, and Pennsylvanian outcroppings; a second and smaller Devonian formation to the north indicates the presence in that period of a separate northern basin. Cambrian and pre-Cambrian rocks, though not exposed anywhere in Indiana, underlie these more recent formations; there are no Permian rocks, because Indiana was above the sea level during and after this period.
Ordovician rocks are exposed only in the southeastern corner because of the uplift that began at this point to form the Cincinnati Arch. Elsewhere in the State Ordovician strata are found beneath more recent formations. Next to the Ordovician outcropping a belt of Silurian rocks is exposed. Farther west the Silurian rocks are overlapped by rocks of succeeding periods -- a narrower Devonian formation, a still narrower Mississippian. Only in the southwestern part of the State are Pennsylvanian rocks found, overlying the uptilted layers of previous periods.
During the many millions of years intervening between the Permian period and the glacial epoch, Indiana experienced three major cycles of erosion. In the entire Mesozoic era, however, the region was above sea level and thus has no rocks of the Triassic, Jurassic, or Cretaceous age. For the same reason no rocks were formed in Indiana during the Tertiary period of the Cenozoic (recent life) era. At the beginning of the Pleistocene (Glacial or Ice Age) the Indiana region was elevated a fourth time. Then came the glaciers, creating by their action many of the salient physical features of present-day Indiana. In the Pleistocene about five-sixths of the whole region -- all except what is now south central Indiana -- was at one time or another under a massive layer of ice, sometimes 2,000 feet thick.
There were at least three ice invasions into Indiana. The earliest, or Illinoian, extended farther south than the Ohio River except in the south central part of the State. Later came the early Wisconsin, which reached a line dividing the northern two-thirds of Indiana from the southern third. The last of the glaciers, the late Wisconsin, covered only the northern half of the State. After each invasion came a warmer period lasting many thousand years, during which the glacier ebbed slowly away, and plants and animals flourished.
The glaciers modified the terrain in several important ways. Their most striking effect was the present bed of the Ohio River, channeled by the ice melting at the edges. They cut off many hills in the northern region, filling the valleys with the rocks thus removed, and smoothing and leveling the entire area. By mixing these materials and grinding them into rock flour an excellent subsoil was formed, particularly a fine clay. Over much of Indiana today the glacial subsoil, the surface of which is excellent farmland, is scores of feet deep, in marked contrast to the shallow and easily eroded surface of the unglaciated areas. Glaciers also greatly altered drainage conditions by destroying streams and valleys, melting and thus creating new ones, and leaving water in many depressions to form marshes and lakes. In melting, they left extensive deposits of sand and gravel they had picked up, and created many hills in the north by piling up soil and rocks into moraines.
The glaciers were not the last agency to alter the surface of Indiana. Wind, water, chemical action, and heat and cold are still at their ceaseless labor of lifting and breaking the soil, cutting into bedrock, and carrying away the debris thus formed. In the unglaciated section the soil is thin and easily worn away, and in the course of centuries innumerable swift streams have cut into bedrock to form deep gorges, canyons, and hills. In the northern two-thirds of the State, however, the processes of erosion proceed much more slowly. Here drainage is less rapid, for the land is level and the streams sluggish.