Page:The Journal of geology (IA journalofgeology11893univ).pdf/157

supplied. In this respect this morainic system contrasts with all other moraines of Ohio, and especially with the later moraines, there being but few bowlders on their surfaces. In eastern Ohio bowlders are a less conspicuous though not a rare feature. The cause of this unusual abundance of bowlders is an interesting problem and one perhaps not easily solved. It has been suggested by some one, I think it was Mr. McGee, that an unusual abundance of bowlders on the later drift sheets may be an indication that the ice invasion which brought them in was preceded by a long deglaciation interval in the gathering ground, and that the Canadian highlands were scoured afresh after the lapse of sufficient time for ledges to have been seamed and broken under atmospheric influence. The suggestion seems worthy of careful consideration.

The drainage from the ice-sheet was especially vigorous at this time throughout the entire width of the state and as far to the east and west as this morainic system has been identified. The altitude could not well have been less than at present, and may have been somewhat greater.

The later moraines. Between this morainic system and the western end of Lake Erie six more or less distinct moraines occur, which were probably formed in comparatively rapid succession. They each consist usually of a broad ridge one or two miles or more in width, and twenty-five to fifty feet in height. They are each sufficiently bulky to have determined to a large extent the courses of the main drainage lines of northern Ohio (see map) and yet they seldom present a sharply indented or conspicuously broken surface. The overwash aprons and terraces connected with them indicate less rapid discharge of waters than from the earlier moraines, and that too in certain parts of the belts where conditions were very favorable for rapid escape of waters as in the north part of the Scioto basin. It is thought from this feature as well as from the aspect of the morainic ridges themselves, that the ice-sheet had less vigor than when forming earlier moraines. That there was a decrease in altitude seems also highly probable. As noted above, surface bowlders