7o6 HERDMAN F. CLELAND 



interesting to speculate upon the time required for the removal of 

 the Lafayette loams, sands, and gravels from the Black Belt and 

 for the reduction of the surface during part of the Pleistocene. 

 Penck's estimate of 500,000 to 1,000,000 years for the duration of 

 the Pleistocene, based upon the rate of advance and retreat of 

 the Pleistocene Ice Sheets, is to be contrasted with Barrell's^ 

 minimum estimate of 1,500,000 years based upon a study of 

 radioactivity. A few years ago Barrell's estimate would have 

 seemed extravagant, but when one considers that a region, such as 

 the one under discussion, has been denuded of a thick deposit of 

 gravel and loam, has been reduced to a peneplain, has been 

 weathered so long as to form a thick residual soil, has been raised, 

 and, finally, has been so dissected by streams as to make a topog- 

 raphy of youthful aspect, the larger estimate does not seem 

 impossible. 



In 1906, Chamberlin and Salisbury^ presented figures as to 

 the duration of time since the Kansan glacial epoch, giving 300,000 

 as a likely minimum, and 1,020,000 as a likely maximum. Had 

 the statement covered the time since the beginning of the Pleisto- 

 cene, these figures would have been considerably larger. 



The physiographic history of the Coastal Plain of the Gulf of 

 Mexico has not as yet been carefully worked out, and it is probable 

 that a thorough study will show that this surface instead of being 

 the youthful topography of a first cycle of erosion, is, for the most 

 part, the incised surface of a peneplain or a plain of marine abra- 

 sion, in which are subordinate peneplains such as that of the 

 Black Belt. The unconsolidated sediments and broad intervales 

 give the impression of youth but the beveled edges of the forma- 

 tions which underlie the Coastal Plain and the level, outstanding 

 cuesta ridges are suggestive of peneplanation. The writer hopes 

 to be able to make a further study of the physiographic history 

 of our Gulf Coastal Plain and with it a study of the Atlantic 

 Coastal Plain. 



^ J. Barrell, "Measurements of Geological Time," Geological Society of America 

 Bulletin, Vol. XXVIII (1917), p. 892. 

 ^ Earth History, Vol. Ill, p. 420. 



