626 THE JOURNAL OF GEOLOGY. 



stream has only partially removed this partial filling of the 

 trench previously cut. The estimated amount of the material 

 so removed since the time of the formation of the kettle moraine 

 is one square mile 650 feet deep, or ^J- as much as the rock 

 excavation. From this it appeared that the amount of erosion 

 in all post-glacial time (including the last of the glacial period), 

 although wrought upon incoherent gravels, is much less than the 

 amount of rock cutting accomplished between the time the river 

 was diverted and the formation of the kettle moraine. 



In the introduction to his paper Professor G. Frederick 

 Wright stated that the hypothesis of an ice dam at Cincinnati 

 appeared to be in a damaged condition, as an agency to account 

 for the high terraces of the upper Ohio and some of its 

 tributaries, and that it was a part of the purpose of the paper to 

 repair the damage. It proved in the sequel, however, an effort 

 at emendation by substitution. The additional facts bearing 

 upon the unity of the glacial period cited in the paper related 

 chiefly to a considerable depth of glacial wash in the trench of 

 a tributary of the Beaver River near Homewood, Pa., just 

 outside but near the border of the glaciated region; Professor 

 Wright contended that the trough in which this glacial material 

 lies must have been eroded previous to its deposition. This 

 erosion he referred to pre-glacial times. The filling reaches 

 nearly or quite to the upper terrace plain on the north side of 

 the tributary, but does not appear on the terrace plain south of 

 the tributary. In the course of his paper, and notably in the 

 discussion following, Professor Wright advanced the hypothesis 

 that the rock shelves which constitute the base of the high 

 terraces of the upper Ohio, Allegheny and adjacent rivers, were 

 formed during a stage of base-levelling in Tertiary times, that 

 the narrower and deeper valley below the rock shelves (in round 

 numbers 300 feet deep) was cut in this base-plane during a 

 stage of elevation just preceding the glacial period, and that 

 this trench was filled up with glacial wash and glacio-natant 

 material to a height, at some points, as much as sixty feet above 

 the rock shelves. In the discussion it was pointed out that, to 



