WARREN UPHAM — DISTRIBUTION OF ENGLACIAL DRIFT. 137 



erosion by a thin covering of subglacial drift. In such cases the glacial deflection 

 lias probably oftener been due to changes in the boundary and slope- of the ice- 

 sheet, ami consequent deviation of its currents during its general recession and 

 departure, rather than to two distinct glacial epochs."' A third proof of subglacial 

 accumulation is the fluxion structure observable at certain planes in many till 

 deposits, indicating that a surface layer of the till was frozen in the bottom of the 

 ice and dragged along over the underlying principal mass, with a shearing move- 

 ment of particle upon particle.t In this way, apparently, the glacial wearing ami 

 striation of the bowlders and pebbles of the till was mostly done. 



Without attempting to review the vicissitudes of the glacial period, its two prin- 

 cipal glacial epochs with their varying stages of ice advance and temporary 

 retreat, and the long interglacial epoch, we see that during the progress of the 

 period all the surface of the bed rock was glaciated, and all the drift material was 

 for some time englacial or at least was grasped and borne forward by the basal 

 portion of the ice-sheet. In its passage across hills and mountains it is easy to 

 understand that the ice, closing up in the lee of the rock highlands, tore from them 

 many fragments, bowlders of large and small size, and rasped off much fine drift, 

 to lie locked in the embrace of the ice as it flowed onward hundreds, or sometimes 

 even thousands, of feet above the 1 surface of the lowlands; and even on many por- 

 tions of the nearly level expanses <>f the St. Lawrence and Mississippi basins 

 eddying convergent and divergent glacial currents doubtless conveyed more or 

 less of the drift upward from the land surface into the lower part of the ice. At 

 Length, when the second or latest ice-sheet attained its greatest extent and thick- 

 ness, much englacial drift was contained within its ma<s, mostly in its lowest 1,000 

 or 500 feet, and extensive deposits of subglacial till lay beneath it. 



Deposition of the Esglacial Drift during the Departure of the Ece-sheet. 



Turning our attention to the ( 'hamplaiu epoch, or time of departure of the latest 

 ice-sheet, let us inquire, as the themes of this paper, What was the manner of 

 deposition of the englacial drift during the final ice-melting? What inequalities 

 are observable in the distribution of the englacial portion of the till? Why is it 

 abundant on some tracts and scanty on others? How was the englacial drift re- 

 lated to the terminal moraines? Can we discover through determinations of the 

 volume of the englacial drift a probable estimate of the time occupied in the 

 accumulation of the moraines? In what forms was the englacial drift left by 

 the departing ice? How much was laid down directly by the ice, and how were 

 other parts modified and unevenly distributed through the assorting, transporting, 

 and depositing action of water in rivers, lakes, and the sea? 



The recession of the Lce-sheet, when warm climatic conditions returned, was by 

 rapid melting upon a considerable breadth, probably 100 to 200 miles or more of its 

 border, which was thus gradually pushed back across all (he drift-bearing area. 

 During the entire summer ami much of the spring and autumn of each year the 

 superficial melting or ablation of the ice produced many rills, brooks, and rivers. 

 Hydrographic basins were thus formed on the ice surface, resembling those of a 



♦ Warren Upham, Geology of Minnesota, vol. i 1884, pp. 505, 549. T. <'. Chamberlin, "The Rock 

 Scorings of the Great ice invasions," I'. S. Geol. Survey, Seventh annual report, for l885-'86, pp. 

 200-207. 



fHugh Uill.T. Reporl of the Fifty-fourth i ting of the Rritish Issoc. for the Idi ol 9i ii qci 



Montreal, 1884, pp. 720, 721. 



Will I'.i i i o,,,i . 9oc. Am., Vol . ::, 1891. 



