INTRODUCTION. 25 



The two great lobes mentioned were best developed at the time of maximum extent of 

 the Labrador ice field, and this apparently occurred at the Illinoian stage of glaciation. At a 

 later stage, the Wisconsin, there were two distinct lobes, the Green Bay and the Lake Michigan, 

 in the western portion of the district covered by the Labrador ice field, and several lobes, the 

 Saginaw, Maumee, East White, Miami, Scioto, and Grand River, within the limits of the eastern 

 or Indiana-Ohio portion. These lobes blended somewhat at the culmination of Wisconsin 

 glaciation. 



The district considered in this monograph includes portions of the Illinois lobe and of the 

 Indiana-Ohio lobe of the Illinoian ice invasion, and the entire Saginaw, Wabash, and East 

 White lobes and the eastern portion of the Lake Michigan lobe of the Wisconsin ice invasion. 

 This Jobation, as long since suggested by Chamberlin, 1 resulted in great degree from the rapid 

 movement and protrusion of the ice in tracts of low altitude. The overlapping of the Labrador 

 ice on ground previously occupied by the western ice indicates a marked change, the cause for 

 which is not yet clearly understood, in the conditions for ice accumulation in the two gathering 

 grounds. 



GLACIAL SUCCESSION. 



The occurrence of several distinct sheets of drift separated by soils and iron-stained, 

 leached, or weathered surfaces has been recognized for some years by those familiar with the 

 glacial deposits of North America. The matter has been discussed by the present writer at 

 some length in each of the two monographs already published, 2 and for this reason only a brief 

 outline is here presented. In order of age from older to younger the following drift sheets 

 and intervals appear: 



OLDEST RECOGNIZED DRIFT. 



In New Jersey and eastern Pennsylvania an old drift, named by Chamberlm and Salisbury 3 

 the Jerseyan and considered by them as old as any yet found in America, is preserved chiefly 

 in places where erosion has been at a minimum and over much of the area is now present only 

 in small detached patches. It is very deeply weathered, a fact that corroborates the testimony 

 of erosion as to its great age. 



In northwestern Pennsylvania 001 the borders of the Allegheny Valley an old drift, pre- 

 served in the recesses of bluffs and in scanty deposits on uplands, seems scarcely so old as 

 the Jerseyan and yet may prove to be its correlative. 



West of the Mississippi an extensive sheet of old drift pertaining to the Keewatin ice 

 field is covered by the Kansan drift and thus has received the name pre-Kansan. It lies also 

 beneath the Aftonian buried soil and weathered zone and for that reason is frequently referred 

 to as the sub- Aftonian drift. Recently Prof. Shimek, of the Iowa Geological Survey, traced 

 it westward into Nebraska and gave it an additional name, the Nebraskan. This drift is 

 separated from the Kansan by the long interglacial Aftonian stage and is much the older, old 

 though the Kansan is, and it is thought to be a possible if not a probable correlative of the 

 Jerseyan drift. It is still, however, not definitely settled that the Labrador and Keewatin 

 ice fields reached the limits of these then oldest drift sheets contemporaneously. 



G. M. Dawson 4 applied the term Albertan to a deposit of mountain drift and gravel in 

 the Province of Alberta in Canada and in neighboring parts of Montana which he considered 

 to be derived from the Rocky Mountain ice. Later investigations by Calhoun 5 raising doubts 

 as to the glacial origin of this gravel have been negatived by W. C. Alden, who in 1911 and 

 1912 discovered glaciated pebbles and distinct evidence of glacial origin. 6 The aspect of the 

 deposit is very aged and at present it is considered an early glacial deposit. 



1 Trans. Wisconsin Acad. Sci., vol. 4, 187S, pp. 201-234; Third Ann. Rept. U. S. Geol. Survey, 1883, pp. 291-402. 



2 The Illinois glacial lobe: Mon. U. S. Geol. Survey, vol. 38, 1899; Glacial formations and drainage features of the Erie and Ohio basins: Mon. 

 U. S. Geol. Survey, vol. 41, 1902. 



3 Geology, vol. 3, 1904, pp. 383-384. 



4 Glacial deposits of southwestern Alberta: Bull. Geol. Soc. America, vol. 7, 1S95, pp. 31-66. 



5 Calhoun, F. H. H., The Montana lobe of the Keewatin ije sheet: Prof. Paper U. S. Geol. Survey No. SO, 1906, pp. 42-52. 

 Pre-Wisconsin glacial drift in the region of Glacier National Park: Bull. Geol. Soc. America, vol. 23, 1912, pp. 687-708. 



