CHAPTER II. 



PHYSICAL HISTORY OF THE BASIN. 



RELATION TO MARINE AND ATMOSPHERIC EROSION AND DEPOSITION. 



It may well be noted that the degree to which shore land basins, such 

 as we are now considering, are developed is in general determined by the 

 amount of time during which a given coast line has remained in about the 

 same position. It is not to be supposed that the coast level remains endur- 

 ingly the same, but rather that in the repeated oscillations the sea does not 

 long desert a given field. During the periods when the area is relatively 

 high the rivers in the lower part of their courses have a chance to develop 

 those wide valleys of gentle slope which are characteristic of regions that 

 have attained very nearly to the general base-level of erosion — i. e., the 

 average position of the sea during its endless variations in height. In 

 general it maybe said that widi valleys next the shore are the best possible 

 indications of a relatively long continued preservation of coastal conditions 

 in the region where they appear. The Atlantic coast of the Americas 

 affords numerous examples of these broad, nearly base-level valleys, which 

 have been formed at divers times in its history. As the existence and the 

 number of these valleys have a distinct bearing on the problem in hand, it 

 is worth while to give a brief general account of them, at least so far as 

 North America is concerned. 



Along the Gulf of Mexico there are half a dozen of these considerable 

 troughs, of which those of the Mississippi and Mobile rivers are the most 

 characteristic, or at least the best known. Both of these valleys, and 

 probably the other basins along this coast, are, at least as regards their lower 

 parts, of relativelv modern origin, dating probably from Tertiary times. 

 On the Atlantic coast, to the north of Florida, there are again a number of 

 these lately formed basins, of which those of Albemarle Sound and Chesa- 

 peake and Delaware bays are the largest and most characteristic. 



