368 Chamberlin and Salisbury — Relationship of 



bills and the eroded edges of the sand and gravel layers, 

 together with their thin covering of local debris, as above 

 indicated, is the mantle of loess. It is absent only where 

 recent erosion has been sufficient to effect its removal. How 

 far the uniformity of distribution of loess over regions of 

 strongly accentuated relief may be due to the creep of the 

 plastic material down the slopes, is not here discussed ; but 

 this is not believed to be an adequate explanation of its uni- 

 form slope distribution. The loess and its clayey equivalents, 

 are confidently believed to have been deposited like a mantle 

 over a previously eroded surface. If the body of the hills be 

 Paleozoic rock with only a capping of gravel and sand, the 

 relation of the loess to the latter is not altered. 



In view of this marked unconformity,, it seems necessary to 

 conclude that the deposition of the gravels and sands must 

 have antedated a long period of pre-loessial erosion, since they 

 themselves suffered erosion, and very extensive erosion, before 

 the close of the first glacial epoch, when the main body of the 

 loess appears to have been deposited. In many places there is 

 evidence that cementation of the gravels and sands of the 

 Orange Sand formation, had been accomplished before erosion 

 had proceeded far. But since cementation may proceed 

 rapidly, it does not seem necessary to allow for any notable 

 interval of time, succeeding the deposition and preceding the 

 erosion of the sands and gravel. Such interval must have been 

 long enough, however, to allow for the shifting of attitude 

 necessary to change the area of Orange Sand, from one of 

 subaqueous deposition, to one of subaenal erosion. 



The force of the above argument is not destroyed, although 

 it may seem to be weakened, by the fact that the relationship 

 between loess and Orange Sand here depicted, does not hold 

 throughout the whole area of the development of the latter. 

 Nowhere else, within the range of the writers' observation, is 

 the unconformity so well shown as in southeastern Illinois. 

 But similar facts are less strikingly exhibited at various points 

 in southeastern Missouri, in western Kentucky, in Arkansas 

 and Tennessee. That the degree of unconformity should vary 

 is not strange, since the attitude of the land was such as to 

 expose it to greater erosion on the flanks of the valley and the 

 highlands of southern Illinois, than in the axis of the valley, 

 where the approach to conformity is greatest. In the south the 

 land appears to have remained so low as to allow of little or no 

 erosion beween the Orange Sand and the loess. 



From a second line of evidence it may be safely concluded 

 that there was an interval of considerable duration between the 

 deposition of the loess and the deposition and exposure of its 

 substratum, 



