2S2 Chamberlin and Leverett — Drainage Features, etc. 



of the region and the general acceptance of some one or another 

 of them will, we feel, be very helpful in drawing interpreta- 

 tions of the Pleistocene history of other regions within the 

 limits allowed by evidence. 



Ratios of work done. — To better illustrate the import of 

 these hypotheses, we may take, as a unit-measure of deposition, 

 the amount of material poured into the . Allegheny trench by 

 the overwash and outwash from the ice while it was forming 

 the adjacent terminal moraines and the drift connected with 

 them, i. e. the last glacial epoch (of this region at least) ; and, 

 as a unit-measure of erosion, we may take all the excavation 

 which these moraine-headed gravels have suffered since their 

 deposition. This last measures not only the work of post- 

 glacial time, but of all that part of the glacial period occupied 

 in the retreat of the ice after it left the rim of the Allegheny 

 basin, when erosion is presumed to have begun by virtue of 

 the withdrawal of the depositing agency. For convenience we 

 will speak of this simply as postglacial erosion. 



Now, the amount of glacial filling contemporaneous with the 

 formation of the moraines and the drift connected with them, 

 assuming that the valley bottom was entirely free from other 

 material, amounts to a section averaging 60 feet in depth and 

 2000 feet in width, as nearly as we can form a judgment from 

 a considerable number of estimates and measurements.* 



The amount of postglacial erosion, estimated liberally, may 

 be represented by a section 30 feet deep and 2000 feet wide.* 



Taking these as units, we present herewith a diagram 

 (Figure 5) which, in the first column, shows the amount of 



DIAGRAM OF 



Ra tios of Work. 



HYP. I. 



Hreli. 



hypJU 



First Glacial 



[Filling 



Inter glavial 



(Exc aval! on ) 

 Gr'auel. Roc/,. Cots. 



Last 

 Glacial. 

 (rutin,) 



POST- 



Glac/al. 



(fxcaoofion) 



1 1 '■ 





■ 

 ■ 

 ■ 



D 



D 

 D 



1 1 



ixxxxxxxxa a 





1 A 





1- 1 tXXXXI A 





First 

 Glacial. 

 (Filling) 



First 

 Inter glacial, 



(Excavation) 

 Gravel. «oci. Cols. 



Second 

 Glac/al. 



{Filling) 



Second 



inter glacial. 



(Excaoafion) 



Gravel. Rock . Cols . 



LAST 



Glac/al 



(Fill/no) 



POST- 



GLAClAL. 



(£xc.) 



Hyp. IV. 



H 





X 





m 



o S 



1 I txvxy^A 



X — Y IXXXX! A 













Fig. 5. . Ratios of work under each of the four hypotheses of glacial filling 

 and excavation. 



filling during the first ice incursion under each of the four 

 hypotheses. This indicates at once the ratio of the first epoch 

 to the last in terms of valley deposition. These ratios are of 



* These are based upon the Allegheny section. 



