DRAINAGE SYSTEMS AKD THE GLACIAL PERIOD. 217 



ter's clay at the level of the railway, where the cut reaches 

 bed-rock, thus proving that the region has been sub- 

 merged." * 



Another crucial case I have myself described at Belle- 

 vue, in the angle of the Ohio and Alleghany Rivers, about 

 five miles below Pittsburg, where the gravel terrace is 

 nearly 300 feet above the river, making it about 1,000 

 feet above the sea. A significaut circumstance connected 

 with this terrace is that not only does its height corre- 

 spond with that of the supposed obstruction at Cincin- 

 nati, but it contains many pebbles of Canadian origin, 

 which could not have got into the valley of the Alleghany 

 before the Glacial period, and could only have reached 

 their present position by being brought down the Alle- 

 ghany Eiver uj^on floating ice, or by the ordinary move- 

 ment of gravel along the margin of a river. Thus this 

 terrace, while corresponding closely with the elevation of 

 those on the Monongahela River, is directly connected 

 with the Glacial period, and furnishes a twofold argu- 

 ment for our theory. 



A still stronger case occurs at Beech Flats, at the head 

 of Ohio Brush Creek, in the northwest corner of Pike 

 County, Ohio, where, at an elevation of about 950 feet 

 above the sea, there is an extensive flat-topped terrace just 

 in front of the terminal moraine. This terrace consists of 

 fine loam, such as is derived from the glacial streams, but 

 which must have been deposited in still water. The oc- 

 currence of still water at that elevation just in front of the 

 continental ice-sheet is best accounted for by the supposed 

 dam at Cincinnati. Indeed, it is extremely difficult to ac- 

 count for it in any other way. 



There are, however, two other methods of attempting 

 to account for the class of facts above cited in support of 

 the ice-dam theory, of which the most plausible is, that in 



* Bulletin of the Geological Society of America, vol. i, p. 478. 



