GENERAL ASPECTS OF ILLINOIAN DRIFT SHEET. 31 



numerous exposures where such rilling reaches a depth of 15 or 20 feet in 

 districts where the general thickness of the gummy clay is scarcely 5 feet. 

 The presence of so many exposures where there is evidence of an interval 

 between the deposition of the till and that of the clay under discussion has 

 led the writer to conclude that in the exposures where the two deposits 

 appear to be blended there has been redeposition of the till in connection 

 with, the later deposit. In so commingled a sheet as till it is a very difficult 

 matter to determine whether redeposition has occurred since the withdrawal 

 of the ice sheet. In view of all the data now available the conclusion 

 seems warranted that this clay is somewhat younger than the Illinoian. 



No suitable name has as yet been found for the clay, although the 

 name gumbo has been applied to it by residents of the region which it char- 

 acterizes, because of its gummy character. This name is open to objec- 

 tion for the reason that it has already been applied to other deposits of 

 different age and different origin. There is a gumbo in the Cretaceous 

 series of the Western plains. The term is also applied to the flood-plain 

 deposits of the Illinois and Mississippi, which are still in process ot accumu- 

 lation. The name gumbo has, however, been used by McGee in his discus- 

 sion of a compact phase of the loess found in southeastern Iowa and 

 northern Missouri, and of a dark clay at its base, apparently the clay under 

 discussion. It is his opinion that the loess there owes its compactness to 

 derivation from the clay beneath it. 1 Until the origin and time relations 

 are more satisfactorily determined, it may be as well to leave unsettled the 

 name for the deposit. 



It remains to consider the probable time relations between the clay 

 under discussion and the sheet of loess that overlies it. The gumnry clay, 

 as noted above, has usually a blackened surface due to humus, a feature 

 which indicates that it was exposed to conditions favorable to plant growth. 

 The plant remains in this clay are seldom sufficiently well preserved to 

 admit of identification. The writer, however, found bits of wood in an 

 exposure along the Santa Fe Railway near New Boston, in Lee County, 

 Iowa, which have been identified by Mr. F. H. Knowlton, of the United 

 States National Museum, as a species of conifer. The specimens were too 

 small and not sufficiently well preserved to enable him to determine the 

 genus and species, though they appear to belong to the genus Picea (spruce). 



i Eleventh Ann. Rept. U. S. Geol. Survey (for 1889-90), 1891, pp. 299, 414, 461-471, 508-510. 



