THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. XYII. — Continuity of the Glacial Period / by 

 G. Frederick Wright. 



Since the appearance in this Journal, for November, 1892, 

 of my paper upon " The Unity of the Glacial Epoch," many 

 important facts bearing upon the subject have come to light, 

 especially in respect to the extent of preglacial erosion in the 

 Ohio Valley. In my report upon " The Glacial Boundary in 

 Western Pennsylvania, Ohio, Kentucky, Indiana and Illinois," 

 published by the United States Geological Survey in 1891,* 

 it was maintained that the erosion of the rocky gorge of the 

 Ohio River and its tributaries was preglacial. In opposition 

 to this view, Professor Chamberlin, in the Introduction accom- 

 panying my report, maintained that the most important part 

 of this rock erosion was wiferglacial. 



To make this point clear it should be stated that the upper 

 portion of the Ohio River and its northern tributaries is char- 

 acterized by a series of gravel deposits containing Canadian 

 pebbles, resting upon rock shelves from 200 to 300 feet above 

 the present river, and from 250 to 350 feet or more above the 

 rock bottom of the river ; while there is a lower series of 

 gravel terraces, rising in places to 130 feet above the river, 

 which is traceable up the streams to the moraine first described 

 by Professor Lewis and myself in 1881. f The relation of 



* Bulletin No. 58. 



f The accompanying map (Fig. 1) to which frequent references will need to be 

 made, shows the 500-foot and the 1000-foot contours of the Ohio Valley ; also 

 the boundary of direct glacial action. The figures give the elevation above the 

 sea. Those along the river refer to the low-water mark. Jn the lower right 

 hand corner is a typical section of the Allegheny showing the rock channel and 

 the high-level terraces. 



Am. Jour. Sci.— Third Series, Vol XLVII, No. 219.— March, 1894. 

 11 



