OUTWASH FROM THE OLDEST DRIFT. 241 



In view of these differences of interpretation Chamberlin and Gilbert made 

 a special study of the terraces on the Allegheny and Monongahela, the 

 results of which have been presented by Chamberlin in a bulletin of this 

 survey/ In this bulletin it was shown (a) that the terraces slope with the 

 present stream, (b) that the material capping the terraces is distinctly fluvial, 

 (e) that the terraces have rock platforms which demand more work than 

 could result from the action of waves in a narrow lake, (cl) that the form 

 and distribution of the terraces are of the fluvial, not lacustrine order, (e) 

 that abandoned channels and oxbows have been preserved in connection 

 with the terraces. The view that the terraces were produced by a lake held 

 in these valleys by an ice dam at Cincinnati seems, therefore, completely 

 set aside by these observations. 



The degree of preservation of tlie gravel on the high terraces jjresents 

 a strikiiig contrast to that of the gravel of Wisconsin age found low down 

 in the Allegheny Valley. It is far more deeply stained and rotten, the 

 difference being as striking as in the respective drift sheets. It also differs 

 from the later gravel in carrying a much smaller proportion of Canadian 

 rocks. Search is often necessary to discover a Canadian pebble in the old 

 gravel, while in the later or Wisconsin gravel the Canadian rocks are a 

 conspicuous ingredient. 



The old gravel is generally fine, and c<5ntains a large admixture of 

 sand, while in places it is interbedded with deposits of clear sand; but, as 

 is natural in river deposits, it also contains a few large stones 1 to 2 feet 

 or more in diameter. These stones are chiefly local sandstones washed 

 in from neighboring bluffs, though occasionally a large Canadian rock is- 

 fomid. 



A few detailed observations concei-ning these deposits and the features 

 along the Lower Allegheny will serve to illustrate the above statements. 



As indicated above, the Allegheny Valley is narrow from Kennerdell 

 to the mouth of the Clarion, and but a small amount of gravel remains in it. 

 These remnants are in every observed case situated on sloping points on 

 the inner curve of sharp bends in the river, and have been cited by Wright 

 as evidence that the valley was excavated to about its present depth prior 

 to the gravel deposition.^ The writer is in agreement with Wright in the 



'T. C. Chamberlin: Bull. U. S. Geo]. Survey No. 58, 1890, pp. 24-32. 

 ^Am. Jour. Sci., .3d series, Vol. XL VII, 1894, p. 175. 



:li 16 



