G. F. Wright — Continuity of the Glacial Period. 187 



might easily have exactly the phenomena repeated which 

 appear at Ackley and at various other places characterized by 

 such moraine-headed terraces as were described by Professor 

 Lewis and myself in 1881. Professor Chamberlin probably 

 needs to revise his interpretation of these facts, in view of a 

 wider knowledge of actual Glacial phenomena.* 



6th. From the time the ice first entered the headwaters of 

 the Allegheny, the Susquehanna, and the Delaware River, the 

 silting up of their channels began. This was effected largely 

 by means of the excessive amount of Glacial debris brought 

 within reach of the streams. But during the earlier retreat of 

 the ice-front from its maximum extent, the silting was facili- 

 tated by the differential northerly depression which existed. 

 During a part of this time, also, it was facilitated in the Ohio 

 Valley by the Glacial dam at Cincinnati. 



7th. After some thousands of feet of ice had melted off, re- 

 lieving the land from a large part of its burden, there-elevation 

 of the continent began ; (and, as probably the most of the sedi- 

 mentation of the preglacial river gorges had been effected dur- 

 ing the earlier portion of this period of recession), there was 

 then an indefinitely prolonged period of re-excavation by con- 

 tinuous torrents of comparatively clear water, facilitated in the 

 Ohio Valley by the wearing away of the Cincinnati dam, which 

 increased by so much the gradient of the stream. 



8th. When equilibrium had been established again, the land 

 was at about its present altitude, but was still covered to a con- 

 siderable depth with ice north of the most prominent moraines. 

 The great size of these moraines is partly due to the vast 

 amount of englacial material held in the lower strata of the ice. 



9th. The deposits of the so-called Champlain epoch near the 

 margin of the glaciated area were considerably earlier in time 

 than those which settled over the Champlain valley itself, since 

 no deposits could take place there until the ice had retreated 

 from the area; but these deposits are properly classed together 

 as Champlain, since they belong to one epoch of general move- 

 ment. 



10th. So great a complication of causes was connected with the 

 production of all the phenomena connected with the period 

 that there were doubtless many oscillations of the ice-front 

 both during the general advance and the general retreat of the 

 ice-sheet. The extent and continuance of these oscillations is to 

 be learned from study of the buried forests and vegetal deposits 

 which lie between the earlier and later sheets of till, and by 

 such instances of erosion as may be clearly proved to be inter- 

 glacial. But there does not seem to be evidence of any oscilla- 

 tions of the front sufficient to break the proper continuity of 

 the period. 



* For an interesting discussion of the points here at issue, see Warren Upham, 

 Bulletin of the Geol. Soc. of America, vol. v, pp. 71-86; 87-100 (Jan., 1894). 



