166 G. F. Wright — Continuity of the Glacial Period. 



run, being wholly devoid of drift material. Possibly this is 

 due to its slightly higher elevation. 



I see no probable explanation of this deposit in Clark's Run 

 except that which implies the preglacial erosion of the Big 

 Beaver down to nearly its present depth ; thus permitting at 

 the same time the erosion of the tributary valley of Clark's 

 Run. Upon the advent of Glacial conditions the wash from 

 the glaciated region brought down the material which, is 

 spread over the rock shelf as far as Clark's Run, and partially 

 filled it. Subsequently the stream has had time only to re-erode 

 a part of the loose material, leaving a portion of it still on the 

 north side of the run. That this rock erosion was very ancient 

 is shown by the size of the gorge in comparison with the 

 stream, and by the Y-shaped character of its mouth. 



Subsequently to my visit, Mr. Leverett looked over the 

 ground, and came to the conclusion that the ice of "the later 

 incursion " had come down into closer proximity to this buried 

 gorge than had been supposed, and hence that the gorge may, 

 after all, have been interglacial.* Still, Professor Chamberlin 

 is compelled to admit that "a portion of the excavation of the 

 rock below the old base-plane may have preceded the incursion 

 of the Glacial wash and even the Glacial period ;" and that 

 " If this should prove true the effect will be to extend [?] the 

 importance of the earlier Glacial epoch and to reduce the 

 time necessarily attributed to the interglacial interval of exca- 

 vation." 



Before the publication, but subsequent to the writing, of this 

 note by Professor Chamberlin, however, Mr. Leverett and I 

 had together gone over a portion of the ground in the Alle- 

 gheny River Valley most likely to yield definite results, includ- 

 ing one of the most important points to which Professor 

 Chamberlin had appealed in support of his theory, and brought 

 to light facts which seem unequivocal in their interpretation. 

 Upon the accompanying map is shown a portion of the Alle- 

 gheny near Warren, Pa., where the Conewango joins the river 

 to the north, and a short run comes in from the south which 

 heads in a low pass separating it from a branch of the Tionesta 

 River, whicli reaches the Allegheny fifty or sixty miles below. f 

 The terminal moraine traced by Lewis and myself crosses the 

 C newango at Ackley, about ten miles above Warren. The 

 elevation of the Creek at Ackley is 1236 feet above tide. The 



* See Professor Chamberlin's note upon the subject in the Journal of Geology, 

 vol. i, p. 628, Sept.-Oct, 1893. 



f Shows details in the Valley of the Allegheny in Warren and Forest counties, 

 Pa. The broken line shows approximately the exterior margin of the rock shelves 

 marking the Tertiary base-level of the stream. The figures along streams give the 

 elevations above tide of the railroad stations which are usually but little above 

 the flood plain. 



