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Rh on both sides of the river. The most significant thing about these high-level terrace deposits is that they contain granitic pebbles^ which are a sure indication that the deposit is postglacial; for none of the tributaries of the Ohio River have access to granite rock, except as fragments have been brought over from Canada by the glacial movement and deposited within their reach.

A sharp discussion concerning the age of the gravel upon these high rock shelves is in progress. On the one hand, Prof. T. C. Chamberlin contends that there have been two glacial periods; that the first period came on before the elevation of land which led to the erosion of the rock gorge, and that therefore this erosion is wholly interglacial, and is evidence of the lapse of an extremely long time between the two periods. On the contrary, I have maintained that the evidence of two distinct glacial periods was insufficient, and have regarded the erosion of this

inner rock gorge as preglacial—that is, as having been effected during the progress of that long period of late Tertiary elevation which culminated in the Glacial period. On this view of the case, the deposits of glacial gravel upon the three-hundred-foot rock shelf have been produced partly by an extensive filling up of the Alleghany gorge as far as Pittsburg and somewhat below, and lower down by the effect of the Cincinnati ice-dam, which set back the water up to this level, and is sufficient to account for many of the facts. Under this view these high-level deposits would coincide approximately with what Dana calls the "Champlain epoch," during which there was considerable depression of land at the north, the influence of which may have been felt as far south as the latitude of Pittsburg.

But whatever may be the difference of opinion about the age of these high-level gravels, there is no disagreement about the glacial character and relatively late age of the lower terraces along the Ohio River such as occur at Steubenville and Brilliant. The rock gorge extends on the average a hundred feet below the present bottom of the river, having been filled up originally by gravel not only to that extent, but to the level of the terrace in which the implement was found. That this extensive deposition