360 W. TJpham — Diversity of the Glacial Dr ij \. 



drift deposits belong to the time of maximum ice advance, 

 and are much earlier than the marginal moraines of thick and 

 irregularly knolly and hilly drift which were accumulated 

 when the ice-sheet terminated farther north. There was, how- 

 ever, in some districts, as Mr. Frank Leverett has found in 

 Illinois, a special massing of the early drift upon a belt near 

 its boundary. In general the drift border is attenuated, but 

 occasionally its thickness in the outermost five miles attains a 

 maximum of 50 feet ; and several times as much drift is found 

 on that belt as in the adjacent drift-bearing belt 5 to 20 miles 

 farther back within the glaciated area. 



In this great region of smoothly spread early till, compris- 

 ing large expanses on both sides of the Mississippi River, 

 there was very scanty glacial erosion of the bed rocks. An 

 area of 16,500 square miles in northeastern Iowa, according to 

 McGee, has nearly everywhere a small thickness of the pre- 

 glacial residuary clay and decaying rock still remaining beneath 

 the universal mantle of the drift, which is principally till, the 

 product of an overriding ice-sheet. McGee further notes that 

 nearly all of the bowlders and smaller rock fragments of that 

 till in both its lower arid upper deposits, and by inference also 

 its finer sandy and clayey matrix, were derived from forma- 

 tions lying north of the limits of Iowa. Bringing much 

 drift, the ice-sheet twice advanced upon this area. Its first 

 advance did not erode even so much as the thin preglacial 

 residuary products of secular rock decay and denudation, 

 which are found to average about seven feet in depth on the 

 adjacent Wisconsin driftless area. Between the two ice incur- 

 sions a forest grew on the land, and its fallen trees and peaty 

 swamps were left upon many townships almost intact, as is 

 known by the forest beds found in digging wells, while the 

 later ice advance covered them with a second sheet of till, 

 which is mostly from 3 or 5 to 10 or 20 feet thick and in some 

 places is probably as much as 80 feet thick. 



The feeble eroding action of the ice-sheets depositing the 

 smooth expanses of the outer and earlier drift is remarkably 

 contrasted with the vigor of erosion displayed by the planed 

 and striated rock surface of the areas enclosed by the later 

 marginal moraines. When the ice-sheets heaped these mo- 

 rainic hills it wore into its adjacent rock bed and accomplished 

 much rock erosion upon all the region of the later and uneven 

 drift, which encloses lakes and lakelets, reaching northward 

 from the outermost large and continuous moraine and cover- 

 ing the far greater part of our drift-bearing area. 



Portions of the Drift Border formed oy Marginal Mo- 

 raines of the Later Drift adjoining the Wisconsin Driftless 

 Area and in the eastern United States. — Along a distance of 



