Page:Popular Science Monthly Volume 33.djvu/442

428 Tizcapa, which furnished the material forming the tufas on which the foot-prints occur, is one of several in the vicinity which have long been extinct, and whose craters are occupied by deep and still lakes. Dr. Brinton's specimen was taken from a quarry on the lake-shore at a point where the overlying strata present a thickness of twenty-one feet beneath the surface soil. These strata comprise five well-marked beds of tufa, beneath which is a deposit of clay, and below this four more beds, with other accumulations in the scams, of pumice and volcanic sand. A heavy deposit of tufa, lying on yellow sand, is then reached. This is the last in the series, and bears on its upper surface innumerable foot-prints—some deeply imprinted, while others are but superficial impressions. As to the age of the foot-prints. Dr. Flint believes the yellow sand under them to be Eocene, but the small shells which it contains are deemed by Prof. Angelo Heilprin to be more nearly Post-pliocene than Eocene. In view of this, and the indications furnished by the overlying strata, Dr. Brinton concludes that there is not sufficient evidence to remove the foot-prints further back than the present Post-pliocene or Quaternary period.

The Lake-Age in Ohio.—Prof. E. W. Claypole has investigated the series of events that occurred in Ohio and the adjoining region during the final retreat of the North American glacier, and has thrown into one view what is known of these occurrences. The terminal moraine of the great glacier crosses the eastern boundary of Ohio a little north of the Ohio River, and extends west and southwest, crossing the Ohio River near Cincinnati. The ice here dammed the river, and ponded back its waters for hundreds of miles. The banks of the Ohio at Cincinnati rise from four hundred to five hundred feet above the water; hence, in order that the ice may have been high enough to pass over into Kentucky, it must have had in the bed of the river a thickness of five hundred to six hundred feet. In this way was formed a lake, which Prof. Claypole calls Lake Ohio, occupying a large tract of the low lands on both sides of the main stream and its tributaries, extending on the north to the edge of the ice-sheet, and hence covering a large share of the southern and eastern parts of the State, reaching beyond the site of Pittsburgh, with arms running up the valleys of the Alleghany and Monongahela Rivers. Lake Ohio must have had a length of four hundred miles, measured in a straight line, and a width of two hundred. Its outlet was probably near Cincinnati, and followed the valleys of the Licking and Kentucky Rivers to that of the Ohio below the ice-dam. When the amelioration of the climate caused the great glacier to retreat northward, there must have come a time when the dam had melted down so that the water could flow over it. A channel was quickly cut in the ice, and the foundations of the dam were undermined. Finally, the dam broke, and all the accumulated water of Lake Ohio was poured through the gap. Days or even weeks must have passed before it was all gone, but at last the lake-bed was dry. When the ice-sheet had been pushed back north of the water-shed which separates the streams that flow into Lake Erie from those that flow into the Ohio River, the water that came from the melting of the ice was held between the front of the retreating ice and the ridge of the land. Thus in the valleys of the Cuyahoga, Sandusky, Maumee, and other north-flowing rivers, triangular lakes were formed with their bases resting against the ice-wall, and narrowing and shallowing back to the water-shed, where they found outlets in south-flowing rivers. All these lakes left monuments behind them in the form of beds of silt, in which are imbedded stones such as might be dropped by floating masses of ice. As the ice-line shrank back down the slope toward Lake Erie, the bases of these triangular lakes spread until they came in communication with each other, and the chain formed one continuous lake, using the lowest of the several southern outlets which had belonged to the separate lakes. As the bed of Lake Erie became uncovered by the glacier, this ancient lake increased in extent, and there is evidence which indicates that when the ice-sheet had retreated still farther the lake formed one vast sheet of water occupying the beds of Lakes Erie and Ontario, the southern part of the bed of Lake Huron, and much of the surrounding country. There was no escape for this water through the St. Lawrence River; that pasagepassage [sic] was still blocked by the ice. Where Fort Wayne, Ind., now stands, there is a gap