G. F. Wright — Unity of the Glacial Epoch. 369 



The second series of terraces connected with the glacial 

 period occur in various oxbows and on numerous shelves of 

 rock bordering the channel at elevations of from 200 to 300 

 feet above the present river level and at about 1,000 feet above 

 tide. Two of the best known localities are Bellevue about 

 five miles below Pittsburgh, and Parker about thirty miles up 

 the Allegheny from Pittsburgh. 



The glacial terrace at Bellevue is 300 feet above the river, 

 and is supported by a shelf of rock about one half mile wide. 

 At Parker the glacial deposits are in a deserted oxbow of the 

 river formed when the level was 200 feet higher than now. 

 Up the Monongahela River there are corresponding terraces to 

 which Professor I. C. White has called attention which are at 

 about the same absolute level, and entirely above the regular 

 river terraces which have been formed in process of the lower- 

 ing of the channel. One of the chief differences between 

 these high-level terraces on the two rivers is that those on the 

 Allegheny have granitic pebbles derived from the glaciated 

 region while those on the Monongahela do not. Professor 

 White describes extensive areas where this mantling of what 

 seems like a lacustrine deposit, consisting of pebbles, and de- 

 posits of clay sometimes thirty feet thick, containing fresh 

 leaves, characterizes the Monongahela up to a level of from 

 1,000 to 1,100 feet and there suddenly cease. 



Professor Chamberlin maintains that these high-level ter- 

 race deposits are merely the remnants of flood plains when the 

 whole drainage level was at that elevation. As a corollary to 

 this he contends that the drainage level was at that height at 

 the time of the first glacial epoch, admitting of the distribution 

 of the granitic pebbles in the Allegheny drift, and that the 

 erosion of the Ohio gorge below that level (i. e. to a depth of 

 about 250 feet for a distance of about 1,000 miles as the water 

 runs) was eroded during the interglacial epoch. To that first 

 epoch he would also attribute the glacial deposits on the south 

 side of the Ohio opposite Cincinnati which 1 had adduced as 

 rendering it probable that there was an ice dam across the 

 river at that point. Professor Chamberlin, on the contrary, 

 supposes these deposits to have been made so much earlier than 

 those a short distance north of Cincinnati that the interval is 

 marked by the whole erosion of the gorge of the Ohio through 

 its whole extent. 



I was not present at the discussion of the Cincinnati ice dam 

 at the Greological Society a year ago. But, from what has been 

 published since, it is evident that the last word has not yet been 

 said about it. Mr. Leverett in particular has attempted to corre- 

 late some of the clay and loess deposits in southeastern Indiana, 

 with deposits of similar character in Illinois, attributing both to 



