THE DRIFT BOEDER. 37 



county line, and there reaches its most northern point in Indiana, It is 

 here that the limits of the portion of the ice sheet properly included in the 

 Illinois glacial lobe should be placed. The boundary from there leads 

 southeastward to the Ohio Valley, and is discussed in another report in 

 preparation. The drift border shows no evidence of an overlapping- at 

 this reentrant angle of one lobe upon territory abandoned by the other, 

 such as was noted on the west side of the Illinois lobe. The border south- 

 east from the reentrant seems to be a direct continuation of that just traced. 



The length of the drift border thus outlined is about 700 miles, and the 

 width of the lobe encircled by it is about 300 miles. The tracing of this 

 border has been the product of several independent surveys. The portion 

 in Wisconsin was largely determined by members of the Wisconsin Geo- 

 logical Survey. The border in northwestern Illinois was partly determined 

 by members of the Illinois Geological Survey, and subsequent] y with greater 

 approximation by Prof. R. D. Salisbury, of the United States Geological 

 Survey, but the precise limits have not as yet been mapped. The portion 

 in Iowa, and also the portion from the southern edge of Iowa southward to 

 the vicinity of St, Louis, have been traced by t!:o present writer. Salis- 

 bury, however, made observations at an earlier date on the limits of the 

 drift in Pike and Calhoun counties, Illinois, and discovered evidence sug- 

 gesting that a portion of these counties is unglaciated. The deposits of 

 drift on the Missouri side of the Mississippi, in the vicinity of St. Louis, 

 were first described by Prof. A. H. Worthen 1 and later by Profs. G. F. 

 Wright, 2 J. E. Todd, 3 and H. A. Wheeler. 4 The portion of the boundary 

 from St. Louis southward to Jackson County, Illinois, was mapped by Prof. 

 G. F. Wright and discussed in Bulletin 58 of this Survey. Wright also 

 mapped the boundary across southern Illinois, but this had previously been 

 outlined with a fair degree of approximation by Worthen. 5 



Wright also made a tracing of the glacial boundary in southwestern 

 Indiana, which was published in Bulletin 58 of this Survey. But the posi- 

 tion of the boundary in that region is found to be shown very inaccurately, 

 the limits of the drift being from 5 to 20 miles outside the limits placed by 



1 Geol. of Illinois, Vol. 1, 1866, p. 314. 



2 Bull. U. S. Geol. Survey, No. 58, pp. 72-73. 



3 Bull. Geol. Soc. America, Vol. V, 1894, p. 539. Missouri Geol. Survey, Vol. X, 1896, pp. 161-163. 

 < Trans. St. Louis Acad. Sci., Vol. VII, No. 3, Feb., 1895. 



6 Geol. of Illinois, Vol. 1, 1866, p. 27. 



