BROWN AND YELLOW LOAM OF NORTH MISSISSIPPI 279 



tions possess little in common. And from such good exposures 

 I have obtained strong evidence of great and widespread uncon- 

 formity between the Brown Loam and all older formations. A 

 great erosion interval is indicated by the following facts : 



1. A considerable interval of erosion between the Lafayette 

 and the Brown Loam periods is indicated by the fact that wherever 

 the two occur in force the greater part of the present surface 

 relief is due to the irregular contours of the Lafayette rather 

 than to varying thickness of the post-Lafayette resulting from 

 recent erosion. In other words, the topographic relief is 

 greater in the Lafayette than in the post-Lafayette. This is 

 indicated by the greater thickness of the post- Lafayette in exist- 

 ing valleys than on hilltops, even where there has been no recent 

 deposition in the former of loam washed in from the hills. 

 .Many Lafayette hilltops, frequently capped with ferruginous 

 sandstone bowlders, seem to have been above water continuously 

 since Lafayette times. It seems that the land in this region has 

 not been under water long enough since the period of Lafayette 

 erosion to allow the complete filling in of the channels cut in 

 the Lafayette ; and this is partly due to the fact that deposition 

 was taking place simultaneously, though not to so great an 

 extent, over the greater part of the hills and ridges into which 

 the Lafayette had been cut. And so the most of our present 

 streams, especially the larger ones, are of the superimposed 

 type — superimposed by sedimentation. 



The greater deposit of sediment in the valleys is probably 

 due to the fact that the valleys were submerged for a longer 

 time, but partly also to the greater effect of their deeper waters 

 in the checking of currents and consequent precipitation of sedi- 

 ments. 



2. Another line of 'evidence of unconformity between the 

 Lafayette and the Brown Loam, closely related to the one just 

 given, lies in the fact that the Lafayette frequently feathers'out, 

 leaving the Brown Loam to rest directly upon the formations 

 older than the Lafayette. Sometimes the evidence of the former 

 extension of the Lafayette over the area in question is not con- 



