272 Chamherlin and Leverett — Studies of the 



Before pursuing the gravel depositions further, a word 

 needs to be said about the glacial formations. 



The Drift. 



The general nature of the drift border of this region is so 

 well known as to need but a passing word. It stretches along 

 the northwestern rim of the present Allegheny basin, en- 

 croaching considerably upon it. It consists of two important 

 factors, the more obtrusive of which is a group of terminal 

 moraines that, to the southwest and northeast, deploy as sepa- 

 rate and independent moraines, but are here so coalescent as 

 to render their individual recognition difficult. It is helpful 

 to bear in mind that we are not dealing with a single terminal 

 moraine, but with a group, and that the more protruding mo- 

 raine in one part is not necessarily the same as in another. 

 We shall have, however, no occasion to dwell upon this 

 distinction. 



Outside of this group of terminal moraines, there is an 

 attenuated drift whose thinness is probably due partly to 

 original deposition and partly to subsequent wastage. To 

 us, it seems demonstrably much older than the term- 

 inal moraines and the drift sheets back of them. Asso- 

 ciated with this attenuated drift, and springing from it, 

 there are trains of gravel. This important fact appears 

 not to have been previously observed. Their connection 

 with it seems to be essentially the same as that of the over- 

 wash and outwash drifts that are so common and signifi- 

 cant dependencies of the distinct terminal moraines. These 

 trains of gravel have, in several instances, been traced to their 

 heads and found to connect themselves definitely with this 

 attenuated border drift at levels much higher than the junc- 

 tion of the moraine-headed terraces with their drift sheets. 

 For example, at the headwaters of Tidioute creek (some seven 

 or eight miles north from the city of Tidioute), there is found 

 a distinct overwash gravel deposit at the outer edge of the old 

 drift at an altitude of 1700 feet A. T., while just above Tidi- 

 oute in the Allegheny valley the early gravels attain a height 

 of 1420 feet A. T., or 330 feet above the present level of the 

 river. This great valley filling was probably from a tongue of 

 ice thrust into the Allegheny below Irvineton as well as from 

 contributions from the tributary, Tidioute creek. 



On one of the headwaters of Pit Hole creek, at Pleasant- 

 ville, similar gravels attain a depth of 80 feet and have an 

 altitude of over 1600 feet A. T. Unassorted drift occurs in 

 immediate connection with these up to a height of 1725 feet 

 (see Carll, Penn. Rept. I, 1875). Similar gravels occur 



