300 R. R. HICE — NORTHWARD FLOW OF ANCIENT BEAYER RIVER 



high enough, perhaps, to entirely cut out the ferriferous limestone. 

 The measured thickness is 150 feet. 



Lake Monongahela 



Before taking up the phase of the north-flowing stage of the Beaver, 

 which is the special subject of this paper, a few words regarding lake 

 Monongahela are in place. Doctor White * has firmly established the 

 existence of this lake, and all study of the subject since the publication 

 of his paper has but confirmed his views. This is among the oldest of 

 our glacial lakes, perhaps the oldest recognized one in the eastern por- 

 tion of the United States. It was synchronous with one of the earlier 

 stages of glaciation (Kansan or pre-Kansan), and therefore any beaches 

 it may have formed and its channels of discharge have had very much 

 longer time to lose their distinctive features than the other glacial lakes ; 

 so that while such records may be found, they have not yet been noted. 

 We are therefore without any recognized beach lines by which we can 

 judge of changes of elevation that may have occurred since the time of 

 this lake. It is possible that a study of delta deposits may aid in such 

 determination, but these will require very careful study on account of 

 their great age as compared with similar deposits in other glacial lakes. 



Abandoned fluvial Plains 



The direction of flow of a stream is clearly shown by the slope of its 

 bed, but in the case of abandoned fluvial plains we may not in all cases 

 be certain of the direction, as the slope may have been distorted by dif- 

 ferential elevation. Where there is no original horizontal line to be 

 found, as apparently in the present case, it may not be entirely clear in 

 which direction the old stream flowed. In the case of the Beaver there 

 is not much difficult}^ of this kind. The slope of the fragments of the 

 old river bed found on the lower Allegheny and the slope on the upper 

 Ohio and Beaver (parallel streams), while quite small, are yet in oppo- 

 site directions, and it is therefore evident that no great diff'erential eleva- 

 tion has occurred, or, if so, it has practically disappeared. Any other 

 supposition necessitates the assumption of a complicated and irregular 

 uplift, entirely out of harmony with the uniformity existing in the re- 

 gion of the Great lakes. 



POT-HOLES IN PRESENT RiVER BeD 



The evidence of pot-holes as to direction of flow is, however, conclu- 



*American Geologist, vol. xviii, pp. 368-379. 



