Pleistocene to P re-pleistocene of Mississippi Basin. 375 



from a time when drainage conditions were such as to allow 

 the rivers to bring in large quantities of gravel, and often of 

 very coarse gravel, from widely different sources. It is there- 

 fore manifest that the attitude of the country must have been 

 very different in the Orange Sand and the glacial epochs, and 

 the shifting of attitudes to match the drainage conditions of 

 these two epochs, may have involved a considerable lapse of 

 time. 



In the light of the foregoing evidence, we find but one con- 

 clusion possible, respecting the age of the Orange Sand. In 

 six states at least, it is true that beneath the loess and above 

 the Orange Sand there is an old surface so deeply weathered 

 and oxidized, as to indicate a long period of exposure before 

 the deposition of the loess. Occasionally this old surface is 

 humus-stained, indicating the growth of vegetation upon it, 

 before the subsequent formation was made. Between this 

 weathered and oxidized or humus-stained surface and the 

 loess, there is wide-spread and often striking stratigraphic 

 unconformity. Everywhere below this horizon, which is 

 clearly recognizable in nearly every one of the thousands of 

 exposures seen by the writers, there is an absence of material 

 which can be referred to a glacial origin, while above this 

 horizon, the loess and other fluvial deposits contain material 

 of glacial derivation.* This old surface, this horizon of oxi- 

 dation, weathering and erosion / this horizon below which 

 glacially derived materials do not occur, and above which 

 they are present, we hold to be the dividing plane between the 

 Pleistocene and the Pre-pleistocene formations. 



Thus far no mention has been made of certain beds of 

 gravel which have given rise to more or less of misinterpreta- 

 tion. So soon as the gravels of the Orange Sand series were 

 elevated above the waters which deposited them, their degra- 

 dation began. From these Pre-pleistocene gravels, materials 

 have been eroded and re-deposited throughout the course of 

 Pleistocene time, and these processes of erosion and re-deposi- 

 tion are still in progress. That there are, therefore, beds of 

 gravel derived from the Pre pleistocene formation in Pleisto- 

 cene times, goes with saying. Such of these beds as were 

 formed in Pleistocene time previous to the deposition of the 

 loess, might well contain glacial pebbles. Such of these beds 

 as were formed during the epoch of the loess, and in the 

 region where the loess was accumulating, necessarily contain 

 something of the later material. In post-loessial times ^too, 



* Microscopic study of these silts and earths cannot be said to have been car- 

 ried to an exhaustive extent. But the statements here made seem to be fully- 

 warranted by the work already done. 



Am. Jouk. Soi. — Third Series, Vol. XLT, No. 245. — Mat, 1891. 

 25 



