316 K. A. DALY OSCILLATIONS OP LEVEL 



distance upstream. Third, a possible but not probable decrease in volume, 

 arising from a change in climate. ... 



"In any case the aggradation of the Allegheny caused every tributary to 

 build up to a gradient over which it was just able to carry its load. The 

 coarseness, slope, and other characters of the deposit indicate that the tribu- 

 taries built up as rapidly as the overloaded master stream. As the stream 

 beds rose they reached the heights one after another of the lowest places in 

 divides between small tributaries, and at such times and places the currents 

 of the rivers were divided and the cols occupied. When final redissection 

 began the rivers took the channels momentarily most desirable, or, if there 

 was little difference in desirability, used both for a time ; thus many parts of 

 valleys were abandoned." " 



The explanation by Shaw and Munn is thus seen to embody elements 

 which had been suggested by Tight and others. In his work on similar 

 drainage changes in Ohio, Tight mentions also crustal warping as a con- 

 ceivable factor, but rejects it If the changes were due to warping, "it 

 would be necessary to assume that after the modifications were well 

 established an exactly opposite warping took place." Such movement 

 "in exactly the reverse order seems extremely improbable." ^^ 



However, according to Jamieson's general hypothesis, just such a re- 

 versal of crust movements in the Ohio Valley ought to be seriously 

 considered. Its probability depends on the relation of the Ohio Valley 

 to the zero isobase for the isostatic movement caused by the associated 

 (Kansan?) ice-cap. Presumably this line lay a little farther south than 

 the zero isobase for crust movements connected with the last glaciation. 

 If the marginal bulge of the earlier glacial stage were located in or near 

 the "Great Valley" of the Appalachian chain, the southern headwaters 

 of the Ohio would have been stimulated and their detrital loads increased. 

 Aggradation in the lower stream courses, outside the upwarped belt, 

 would thus result from a cause which is additional to the causes listed by 

 Shaw and Munn. With sufficient aggradation, the rivers would locally 

 find, on solid rock beyond their valley walls, channels lower than their 

 own alluvial plains. This local abandonment of the old channels would 

 cease when the Great Valley belt subsided as a result of deglaciation. 

 Most of the new channels would, however, be permanent. 



Even from the brief review given, it is obvious that the testing of 

 Jamieson's hypothesis in the Ohio Valley portion of the marginal belt is 

 an exceedingly complicated problem. The writer's only excuse for ventur- 

 ing so far into speculation is to open the subject for discussion by those 

 more familiar with this extensive field. Drainage peculiarities of some 



18 E. W. Shaw and M. J. Munn: Burgettst own-Carnegie Folio (no. 177), U. S. Geol. 

 Survey, 1911, p. 9. 



19 W. G. Tight : Professional Paper No. 13, U. S. Geol. Survey, 1903, pp. 93, 97. 



