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



this portion of the rock gorge to Glacial movements is one of 

 the principal points in controversy. My contention has been 

 that this part of the rock gorge was preglacial ; while in the 

 Introduction referred to, Professor Chamberlin states (p. 35) 

 his position as follows : 



" The higher Glacial gravels antedated those of the moraine- 

 forming epoch by the measure of the erosion of the channel 

 through the old drift and the rock, whose mean depth here is 

 about 300 feet, of which perhaps, 250 feet may be said to be 

 rock. The excavation that intervened between the two "epochs 

 in other portions of the Alleghany, Monongahela, and Upper 

 Ohio Valleys is closely comparable with this." 



In this Journal, for March, 1893, in a paper replying to 

 mine of the previous November, Professor Chamberlin reit- 

 erates in the strongest manner his adherence to his previously 

 expressed views concerning the interglacial date of this rock 

 erosion in the Ohio Valley. Thus he writes : 



" These terraces I have maintained* were produced at a 

 time of base level degradation, which in its later stages was 

 contemporaneous with the earlier ice incursion whose water 

 bore gravels down the Allegheny, Upper Ohio and some adja- 

 cent streams, and formed the 40 or 50 feet of capping which 

 lies upon the rock benches that constitute the body of the 

 terraces. I have argued that subsequent to this the land, was 

 elevated and the lower newer steep-sided gorges of the Alle- 

 gheny and neighboring streams were cut to a depth that may 

 be roundly stated as 250 feet, and that subsequent to this the 

 later ice incursion formed the outer moraine of the region. 

 From the outer side of this moraine Glacial streams bore their 

 sands and gravels down the Allegheny gorge cut during the 

 interglacial interval. The evidence of this, gathered in a 

 joint study by Mr. Gilbert and myself, may be found in 

 Bulletin 58, IT. S. Geol. Surv., pp. 32-36. I therefore argue 

 that between the time when the Glacial gravels were deposited 

 on the high terraces and the incursion of the later ice there 

 was a cutting of the gorge to the depth of 250 feet roundly 

 speaking. I regard this gorge-cutting as a minimum measure 

 of the interval between the two ice incursions." 



During the past year I have devoted as much time as I 

 could spare from other duties and other fields of investigation 

 to the collection of additional facts bearing upon the solution 

 of this important problem. In December, 1892, under guid- 

 ance of Mr. Richard E. Hice, of Beaver, Pa., I examined the 

 valley of the Big Beaver between the Ohio and the Glacial 

 boundary, with the result of finding, just below the line of 



* Bulletin 58, U. S. Geol. Surv., pp. 20-38. 



