124 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



material may be assumed to have been inevitable. This is dependent upon the same 

 principle of action that would permit the retention of gravels in such situations if 

 they had been previouslj'- deposited within the trench. 



As the gravels on the slopes are usually thin sheets or patches, we have not 

 found decisive evidence, in themselves, as to whether they are remnants of earlier 

 gravels, or incidents of degradation. We have searched industrious!}- for evidence 

 that should be decisive on this point. Such evidence should be found in abandoned 

 segments of the old valley, if it had been deeply excavated before the deposit of the 

 gravels and had subsequently been filled by these up to the summit of high gravels. 

 These high gravels fill oxbows and recessed shelves, and the stream which deposited 

 them had, in manj^ instances, alternative courses. This is notably true in the vicinity 

 of Pittsburg. Here the old high plain of rock was extensivelj" covered bj^ the waters 

 that deposited the gravels, as is shown by the presence of remnants. There are, in 

 the eastern part of the citj^, four islands surrounded hy broad channel ways, among 

 which the waters distributed glacial gravels in greater or less degree. Now, if the 

 present deep Allegheny and Monongahela trenches had been cut previous!}^ to the 

 filling in of the gravels, there is only a small chance that, after the gravel-depositing 

 period, during which they were flowing 50 feet or more above the rock plain, thej' 

 would have descended the second time on precisely the same lines. Between the 

 several broad channels open to them the possible combinations are 32 in number, and 

 hence theoretical Ij^ the chances of a combination repeating itself are one in 32. If 

 it be objected that certain of the courses are more favorably situated than others, our 

 answer is, first, why were these others then ever produced by the streams or occu- 

 pied by glacial wash; and our second answer is, that if this be true of certain combi- 

 nations, it does not seem to us to be at all true of many others. 



Besides this, along the Alleghenj' River above, and also along the Ohio River 

 between Pittsburg and Toronto, to which point the high gravels containing Canadian 

 pebbles have been traced, there are perhaps a score of oxbows, deep recesses, 

 shelves, or available cols which would afl'ord opportunities for the redescending 

 river to locate itself on other lines than its old track with all its meanders. When 

 these possibilities are added to the preceding it becomes exceedingly strange that, 

 below the mouth of the Clarion, no abandoned channel is found which retains any 

 old filling comparable in depth to the present trench. We find numerous channels 

 containing gravels ranging from .50 to a little over 100 feet that represent such old 

 coui'ses on the higher plain. This demonstrates the truthfulness of the principle 

 here urged, and shows its application to this particular field. 



The hypothesis that the river trenches of this reg-ion had been cut to 

 essentially their present depth before the earliest glaciation encounters 

 another serious difficulty in that it calls for a greater amount of valley filling 

 than can well be postulated. It necessitates enough valley gravel during that 

 glaciation to produce a filling fully 300 feet in depth for a distance of at least 

 250 miles, and that, too, while the ice edge occupied the narrow belt between 

 the glacial boundary and the basin of Lake Erie. The amount of material 



