136 PROCEEDINGS OF WASHINGTON MEETING. 



it with the drift material contained in its basal portion under the weight of thou- 

 sands of feet <>f ice. The large proportion of limestone present in the sand and 

 finely powdered rock of the drift in regions of limestone formations demonstrates, 

 as Chamberlinhas shown, that the drift was chiefly derived from glacial wearing 

 of the bed-rocks* It should be added, however, that the depth of the glacial ero- 

 sion was probably nowhere so great as to change the principal and grander topo- 

 graphic features of the preglacial contour. The most important influence of glacial 

 action upon the topography was usually the removal or partial wearing away of 

 comparatively small projecting knobs and the filling up of depressions and valleys. 

 bringing the surface to a more uniform contour than before the ice age. 



If the rocks underlying the drift have been so universally glaciated, losing their 

 preglacial mantle of loose superficial deposits and further suffering almost every- 

 where much abrasion by the ice-sheet, it is evident that at some time between the 

 beginning and the end of the glacial period every part, even every square rod. 

 with rare exceptions, lias been subjected to grinding and rasping by the ice and 

 its enclosed drift. The thickest drift deposits when removed are found to have 

 rested on firm and sound rock, which bears no trace of preglacial weathering, but 

 is planed and striated by ice-wearing. We therefore must conclude that earlier or 

 later all of the drift has been plowed up and borne forward within the ice, or 

 pushed and dragged along beneath it, strongly held in the grasp of the bottom of 

 the ice. Any mode of action which seems consistent with the observed glacial 

 erosion of the rock floor would require intermingled ice and drift to lie swept 

 over it during some part of the glacial period. All the drift therefore has been at 

 some time essentially englacial, being transported while embedded in the ice. 



But the masses of till forming slopes upon higher hills of rock, sometimes on 

 their lee side, sheltered from the ice-current, but often on the stoss or exposed 

 side, and not rarely in both situations, then almost enveloping the rock hill, were 

 evidently deposited beneath the ice-sheet as subglacial till or ground moraine, and 

 afterward remained undisturbed in their present place while the ice-current con- 

 tinued to flow over them. Indeed, the position and character of these slopes of till 

 prove that they must have been gradually accumulated by the addition of drift 

 which had been englacial and became lodged on their surface. The massive 

 hills of till, round, oval, or elongated, which are called drumlins, have many 

 features analogous with the slopes of till just noticed, and like them are doubt- 

 less subglacial accumulations consisting similarly of drift that was formerly engla- 

 cial, amassed in these hills by gradual accretion. Lower tracts of till, also occu] ly- 

 ing the greater part of the drift-bearing area, show by their composition, hardness, 

 and obscure lamination that they were subglacial deposits. 



Among further proofs of the accumulation of much of the till under the ice, as 

 its ground moraine, are bowlder pavements, where a surface of till has been evi- 

 dently planed and its bowlders striated by glacial erosion. Again, one portion of a 

 rock surface has been occasionally planed and striated by a glacial current moving 

 in a different direction from that which similarly eroded an adjacent portion of 

 the same ledge; and the two areas often have different slopes, their line of meet- 

 ing being a beveled edge. < me part of the rock has been protected from the later 



*U. S. Geol. Survey, Third Annual Report, p. 312; and Sixth Annual Report, memoir by T. C 

 Chamberlin and R. D. Salisbury, " The Driftless Area of the upper Mississippi," pp. 241, 247, 255 ; 

 also Am. Jour. Sci., 3d series, vol. xxvii, 1884, p. 388. Compare" Composition of the Till or Bowlder- 

 clay," by W. o. Crosby, Proc. Boston Soc. of Nat. Hist., vol. sxv, 1890, pp. 115-140. 



