98 W. UPHAM — THE SUCCESSION OF PLEISTOCENE FORMATIONS. 



of the ice-sheet and accompanying its departure up to the time of for- 

 mation of the great marginal moraines, testifies that previous to the 

 farthest glacial advance the land sank to its present altitude, and prob- 

 ably somewhat lower on the area of the early drift, but not to the sea 

 level. The vast weight of the continental glacier seems to have been the 

 chief or only cause of this subsidence, as was first pointed out by Jamie- 

 son for the similar depression of the British Isles and Scandinavia at the 

 time of final melting of the European ice-sheet. The explanation of 

 this continuance of the ice accumulation and advance after the depres- 

 sion of the land began and until the maxima both of the land subsidence 

 and ice extension were attained, with a low altitude and even less de- 

 scent of the lower Mississippi than now, has been well given by Le Conte 

 in a paper before this Society.* The subsidence was doubtless slow, 

 even though probably many times faster than the preceding uplift. It 

 may have occupied only 5,000 years, being at a yearly rate of a half a 

 foot to one foot; but possibly it was two or three times as long. While 

 the slow sinking of the land was taking place, the accumulation of the 

 ice by snowfall may have proceeded at a somewhat more rapid rate, so 

 that the thickness of the ice-sheet and the altitude of its surface were 

 increasing up to a maximum nearly coincident with that of the sub- 

 sidence. Finally, however, the subsidence brought a warmer climate 

 on the southerii border of the ice, causing it to retreat, and giving to it 

 in the region of the marginal moraines a mainly steeper frontal gradient 

 and more vigorous currents than during its growth and culmination. 



Ke-elevation during the Departure of the Ice-sheet. 



When the ice-melting relieved this part of the earth's crust from its 

 burden, and as fast as the ice-sheet receded, there followed a re-elevation 

 of the land to nearly its present height. First at the south the region 

 of the loess and of the driftless area in Wisconsin and adjacent states 

 was uplifted so that the rivers speedily eroded much of their recently 

 deposited fine glacial silt. The streams discharged from the ice-border 

 during the formation of the moraines mostly flowed with strong cur- 

 rents, laying down gravel and sand beds along the bottoms of valleys 

 bordered by high terrace remnants of the loess. Later the area of lake 

 Agassiz was uplifted, causing its earliest upper beach to have a gradual 

 ascent averaging about one foot per mile along an explored distance of 

 400 miles from south to north ; and this uplift was rapid, being nearly 

 completed before the formation of the lowest and latest beaches of this 

 lake, which are almost or quite horizontal. The central and northern 



* Bulletin G. S, A., vol. ii, 1891, pp. 329, 330, Compare Le Conte's Elements of Geology, third edi 

 tion, 1891, p. 589, 



