184 R. D. SALISBURY — EXTENSION OF PKE-PLEISTOCENE GRAVELS. 



The attempt was long since made by the writer, under the direction of 

 President T. ('. Chamberlin, to determine the stratigraphic relationship 

 between the glacial drift and the "Orange Sand" gravels of southern 

 Illinois and the contiguous areas of Missouri, Arkansas, Kentucky and 

 Tennessee. That portion of southern Illinois occupied by the southern 

 margin of the drift is the area which has been especially studied in the 

 hope of finding these two formations in contact, and therefore in such 

 relationship as to determine their relative age. It was known that the 

 " Orange Sand " gravels * extended northward to within a few miles of 

 the glacial drift. Their distribution in the northern part of their exten- 

 sion was known to be much interrupted by erosion, and it was the hope 

 that certain areas of the gravel might be found as far north as the south- 

 ern limit of the drift; but up to the present season it had seemed that 

 the southern gravels failed to reach the drift-covered territory by twenty 

 or twenty-five miles. 



Discovery of "Orange Sand'' within the Drift Limits. 



Ancient Gravels replacing Drift. — In May and June of the present sea- 

 son, what appears to be a small driftless area was found to exist in 

 Pike and Calhoun counties, Illinois.!" In this area, apparently free from 

 northern drift, the loess was found to be underlain by an interrupted 

 bed of gravel of variable thickness, corresponding to the " Orange Sand " 

 gravels farther southward. The gravel is found mainly on the level up- 

 lands and on the summits of ridges where erosion has been least. It was 

 thus determined that the gravel formation hitherto known only south of 

 the northern drift had a northward extension much beyond the south- 

 ern border of the ice-sheet; but the stratigraphic relationship of the drift 

 and of this gravel was not directly shown by the new find, though the 

 occurrence of the gravel in this situation — in an area completely sur- 

 rounded by brift — tended strongly to confirm the previous conclusion as 

 to its pre-Pleistocene age. 



Ancient Gravels underlying Drift. — Subsequently the area surrounding 

 the newly found driftless tract was studied, and in northern Pike, in 

 Adams, and in Hancock counties gravel identical with that in the drift- 

 less area of Calhoun and Pike counties was found to exist. In these 

 counties its position is such as to indicate unequivocally its relationship 

 to the glacial drift. Wherever it is seen in section in these counties it 

 constitutes a well defined layer inferior to the till. 



* The term " Orange Sand " gravels is here used in its widesl sense, including all thai has been 

 designated by this term. 



f'On the probable existence of a second driftless area in the basin of the Mississippi river." 

 Read before the Am. Assn. Adv. Sci., Section E, 1891. 



