and Deposits of Flooded Rivers. 39 



active during the Pliocene period, has produced the main 

 features of the present valleys of the Mississippi and Ohio 

 Rivers ; and in the latter the smoothly flowing outlines and 

 moderate slopes of the surface shaped by this erosion present a 

 remarkable contrast with the steeper sides of the inner or 

 lower part eroded during Quaternary time. 



The highest gravel terraces of the Ohio are found at the 

 line thus dividing the Tertiary and Quaternary erosion, and 

 these terraces contain material supplied from the drift of the 

 first Glacial epoch.* The ice-sheet of this epoch stretched 

 south to the northern tributaries of the Ohio, and even crossed 

 this river at Cincinnati, probably damming back its waters for 

 some short time, and perhaps repeatedly, to 'form a lake along 

 the Ohio to Pittsburgh and far beyond up the confluent Alle- 

 gheny and Monongahela valleys. A glacial dam could exist, 

 however, only at the stage of maximum advance of the ice, 

 and its short duration permitted little record in terrace or delta 

 deposits. The high terraces before noticed are of fluvial for- 

 mation, apparently belonging chiefly to the time of the reces- 

 sion of the earlier ice-sheet, when the floods supplied by its 

 melting were discharged along this valley, bringing part of its 

 englacial drift and laying it down as a gently sloping fluvial 

 plain of gravel, sand and silt. 



On the Ohio, Allegheny, Susquehanna and Delaware Rivers, 

 below their highest terraces and scanty remnants of the flood- 

 plains of the first Glacial epoch, Chamberlin and McGee find 

 that interglacial erosion deepened the valleys 200 to 300 feet 

 through the hard bed-rocks before the epoch of the later 

 glaciation.f The drift of this second Glacial epoch is dis- 

 tinguished from that of the first, which has a greater extent in 

 the Mississippi basin, by terminal moraines formed on the ex- 

 treme boundary of the later ice-incursion and at successive 

 stages of halt or re-advance of the ice-sheet during its depar- 

 ture. A long interglacial epoch, as measured by years, must 

 have divided the epochs of glaciation, and its cause was proba- 

 bly a considerable subsidence of the continent, or at least of 

 the eastern United States and the Mississippi basin, but appar- 

 ently not to the sea level. That there was greater altitude, 

 and probably more descent from north to south during the 

 interglacial epoch than during the Tertiary erosion, is indicated 

 by the narrowness and steep sides of the Quaternary portion 

 of these valleys. 



From the observations thus made in valleys tributary to the 

 Atlantic ocean and the Gulf of Mexico, the earliest Quaternary 

 record for the northern part of that area appears to be the 



* Chamberlin and Wright, Bulletin No. 58, U. S. G-eol. Survey, 

 f Bulletin Geol. Soc. of America, vol. i, pp. 472-4. 



