A PLEISTOCENE PENEPLAIN IN THE COASTAL PLAIN 705 



that the Black Belt was once covered with it. The almost complete 

 absence of the Lafayette over the area underlain by the Selma chalk 

 and its presence on other parts of the Coastal Plain north and south is 

 attributable to the greater ease with which the chalk is weathered 

 and eroded. Because of its solubihty and lack of strength, the 

 streams that flow through the Umestone quickly cut their beds to 

 grade. In other parts of the Coastal Plain which are underlain 

 by limestone, it is also found that very little remains of the once 

 widespread cover of Lafayette. 



The features which lead to the belief that the Black Belt of 

 Alabama is in the youthful stage of a first cycle of erosion was 

 based upon the facts (i) that its surface is so level in certain areas 

 as to give it an appearance of topographic youth; (2) that the 

 rivers are bordered by steep banks or bluffs and are in a youthful 

 stage of an erosion cycle. 



The evidences which indicate that the region was peneplained 

 and has been elevated in comparatively recent times are: (i) that 

 it occupies a troughlike depression 200 to 300 feet lower than the 

 bordering lands to the north and south; (2) that, although the soil 

 is a clay, and is consequently very favorable for the retention of 

 water, swamps are nevertheless uncommon except in river bottoms, 

 showing that the drainage had been thoroughly established; 

 (3) that the Lafayette, which once covered the Black Belt, has 

 been almost entirely removed from it; (4) that the thick, residual 

 soils of the region were probably formed chiefly after the land was 

 reduced to a peneplain (at the present time they are being 

 rapidly eroded. away); (5) that the present youthful appearance 

 of the region is due to a comparatively recent elevation of the 

 peneplain 60 or more feet, which permitted the rivers to sink 

 their beds; (6) that the peneplanation must have taken place 

 during the Pleistocene, as is shown by the fact that the region 

 was reduced to a nearly level surface and that a thick residual 

 soil was formed after the removal of the Lafayette, a formation 

 that was deposited not earlier than late PHocene and, more prob- 

 ably, during the Pleistocene. 



Estimates of the length of geological time are so uncertain that 

 little dependence can be placed on them, but it is, nevertheless, 



