Page:Popular Science Monthly Volume 38.djvu/762

742 great size. None of the tributaries of the river, either from the north or the south, flow through regions more recent than the Carboniferous, with the exception of the lower parts of the Ohio itself and of the Tennessee, which border on the Quaternary. The lowest formation in the valley is the Cincinnati, which is just touched at a single point, and only for a short distance, about twenty miles above the city.

It may be stated, then, that since the close of Carboniferous time the river has flowed mainly in the same channel. The vast antiquity of the river is thus easily established, and the existence of the wide valley, with its broad bottom lands, is readily accounted for. The story of the river during the long period of pre-glacial time would be simple. For ages its waters were probably poured directly into the Gulf of Mexico, an arm of which extended northward into the continent at least as far as the present site of Cairo, Illinois. In later time the Mississippi-Missouri began the formation of a delta, which, gradually extending, has left the Ohio a tributary merely of the mighty "Father of Waters." As ages passed away it smoothed its rocky bed, and cut deeper and deeper between the hills, until at last there came a time in the history of the earth which man has called the "Glacial period." It was an age of intense cold—when a mantle of ice and snow covered all the New England States, New York, part of Pennsylvania, of Ohio, of Indiana, and Illinois, and thence extended northwestward to Dakota and the Rocky Mountain region. When the period was at its height, and the maximum limit of the ice-sheet had been reached, the course of the Ohio River became seriously affected.

Profs. G. F. Wright and H. Carvill Lewis, Mr. Warren Upham and others, have shown that, at the period of the greatest extension of the ice, a portion of it crossed the Ohio River in the vicinity of Cincinnati, and extended southward for some miles into Kentucky. The course of the river as it now exists was blocked for a distance of probably fifty miles, or from near Point Pleasant, twenty miles above Cincinnati, to the mouth of the Big Miami, thirty miles below.

Investigations into the topography and surface geology of the region about Cincinnati reveal the existence of an ancient channel of the Ohio which divided into two branches. One was on the eastern, the other on the western side of the city, and the two united just north of the city and continued to Hamilton, twenty-five miles. Here the old stream was joined by what is now the Big Miami, and the united rivers then turned southwestward and