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



In the present state of evidence as made known to the 

 general scientific public it is difficult to discuss the progress of 

 events since the beginning of the glacial period in the lower 

 Mississippi Yalley. But according to Professor Chamberlin 

 what is called the Orange Gravel and has heretofore been 

 regarded as of glacial age, -is preglacial. It does not contain 

 granitic pebbles. His study of the deposit also convinces him 

 that it was originally continuous over the entire trough which 

 has a width of about sixty miles (Bull. Geol. Soc. Am. p. 471). 

 The only relic of the earlier glacial deposits which he would 

 recognize over this area is the silt which everywhere caps the 

 Orange Sand and extends beyond it to the east. This he 

 would connect with the loess in the upper part of the valley 

 extending to the moraines. Apparently he does not regard 

 this silt as a deep water deposit but as a deposit spread out over 

 a vast flood plain when the drainage was slack both from a de- 

 pression northward and from the previous filling up of the 

 trough by the Orange Sand. But granting that this is the cor- 

 rect interpretation of the phenomena, it is difficult to see how 

 the erosion is shown to be "interglacial." If that depressed 

 condition of things occurred at the climax of the period, why 

 may not the erosion simply gauge the time since that climax? 

 I will, however, leave others more familiar with the facts to 

 discuss the various assumptions underlying the argument from 

 the Lower Mississippi, and turn to the problem of erosion and 

 deposition in the valley of the Ohio River of which I have 

 more personal knowledge. 



In brief the facts are that the Ohio occupies a trough, from 

 three to five hundred feet in depth and averaging from a 

 half mile to a mile in breadth, which has been eroded from 

 nearly parallel strata of Palseozoic time. Nor does the present 

 depth of the trough represent the whole extent of erosion. 

 The channel is filled with gravel to a depth of from fifty to 

 a hundred feet or more below the low water mark. Two 

 sets of terraces containing granitic, and therefore glacial, drift 

 mark this trough all the way from Louisville to the head- waters 

 of the Allegheny River up to a level between 1,000 and 1,100 

 feet above tide. The lower terrace is continuous and is definitely 

 traced up all the northern tributaries to the terminal moraine 

 and is much higher and coarser near the moraine, and wherever 

 tributaries come into the Ohio from the glaciated region, than 

 it is below. This terrace rises at its highest points to about 

 120 feet above low water mark, that being the height at Cin- 

 cinnati and at the mouth of Beaver Creek in Western Penn- 

 sylvania. Evidently the channel was at various places origin- 

 ally filled up to this height so as to raise the water level by 

 that amount all the distance above Cincinnati. 



