PRE GLACIAL DRAINAGE. 299 



out the valleys, and silting up the sea. The Alleghany 

 Mountains were at one time the bed of the ocean upon 

 which this sediment was deposited. The sandstones, shales, 

 and conglomerates of the coal-measures attest the activity 

 of the forces of that early period. The tops of the mount- 

 ains in southern New York and northern and eastern Penn- 

 sylvania are covered with subcarboniferous conglomerates of 

 almost incredible depth and extent, consisting largely of 

 well-rounded quartz pebbles, of all sizes up to two or three 

 inches in diameter. These are water-worn, and must have 

 been rolled along by impetuous currents from far-distant re- 

 gions. Thus the tributaries of the Mississippi are, at the 

 present time, bringing into its valley similar deposits from 

 the mountain plateaus on either side. But no sooner did 

 the convulsive forces of the earth begin to lift this great, 

 stratified ocean-bed of the Appalachian region above the 

 water, than it too became the subject of erosion, and began 

 to furnish material for newer deposits farther to the south 

 and west. 



The Ohio River and its tributaries furnish a good exam- 

 ple of the extent of preglacial erosion. The traveler is im- 

 pressed with the gorge in the Niagara River below the falls, 

 as showing the force of running water when concentrated in 

 a single line of drainage. The gorge of the Niagara, how- 

 ever, is only about seven miles long, a thousand feet wide, 

 and three hundred feet deep. This, as will appear later,* 

 is one of the best measures of post-glacial erosion. But the 

 Ohio River, containing a far less volume of water, has worn 

 a much larger and deeper trough more than a thousand miles 

 in length. The character of the trough of the Ohio and its 

 tributaries is readily discerned, even by the passing observer. 

 The strata on the opposite sides are horizontal, and match 

 each other like the ends of a plank that has been sawed asun- 

 der. The alternate layers of conglomerate, coal, shale, and 

 sandstone upon the one side of the river correspond to simi- 



* See Chapter XX. 



