Page:The Journal of geology (IA journalofgeology11893univ).pdf/156

moraine lying east of Mad river. It is difficult to determine the precise amount of excavation accomplished in this interval, since the portions of the valley lying beneath or within the later moraine are partially filled, while the portions lying outside the moraines afforded avenues for the escape of glacial waters, and were probably much enlarged thereby. It seems safe, however, to state that an amount of excavation took place that would require some thousands of years with a drainage system of the size of the present Mad river system, and with a gradient such as the region now affords. It may be added that in regions further west, if our correlations are correct, there are found evidences of the same deglaciation interval, but their discussion does not fall within the scope of this paper.

Main morainic system of later drift. The moraine just referred to (in whose re-entrant angle the Mad river basin lies) belongs to the system mapped and described by Professor Chamberlin, in the Third Annual Report of the U. S. Geol. Survey, as the "Terminal Moraine of the Second Glacial Epoch." As shown by Professor Chamberlin this moraine lies near the glacial boundary in eastern Ohio and north-western Pennsylvania, but farther west it falls short many miles of reaching the glacial boundary. It is a complex system, "constituting a belt rather than a single moraine," there being in places not less than four distinct members. Nearly everywhere in the state it presents a sharply indented surface, a feature which, as suggested by Professor Chamberlin, appears to indicate forceful or vigorous action of the ice-sheet. Its peculiarly sharp contours and their diagnostic characters make it the most conspicuous and distinctive morainic belt in the state. Other moraines, newer as well as older than this morainic system, assume in places the form of smooth ridges or have but gently undulating surface, and hence are less conspicuous features even where they have as great bulk as the individual members of this system. In western Ohio one of the members of this system (the second one of the group) carries on its surface large numbers of crystalline bowlders of Canadian derivation and the remaining members are liberally