1 68 SEA AND LAND 



Bays, Pamlico and Albemarle Sounds, the harbors of Charles- 

 ton and Savannali, and Mobile Bay afford perhaps the best 

 instances of this class of havens. At the time when the sub- 

 sidence which produced these reentrant delta harbors took 

 place, the valley of the Mississippi was occupied by the 

 greatest of these bays, which probably extended for several 

 hundred miles above New Orleans and had at some points the 

 width of fifty miles or more. Owing to the very large amount 

 of detritus which that stream bears to the sea, the whole of 

 the reentrant has become tilled by the newly formed delta, and 

 the accumulation now projects beyond the ancient valley and 

 thus has the normal form of such deposits. 



It is characteristic of the ports which are formed in these 

 flooded valleys, that they were originally wide-mouthed, and 

 narrowed gradually toward their head waters. In many in- 

 stances the passage from these basins to the sea has become 

 constricted by the formation of sand spits, built in the manner 

 hereafter to be described. Owing to the plentiful incursion 

 of river waters heavily charged with sediment, the harbors of 

 this group are apt to become shallowed, and, except so far as 

 they are kept open by tidal currents, are often unfitted for the 

 use of large ships. This shallowing of the water is likely to 

 be most conspicuous in the parts of the basin near the existing 

 mouth of the true river. It occasionally happens, that, while 

 the floor of the bay over the greater part of its extent has 

 been converted into shoal water, some of the side gorges 

 originally occupied by small streams retain a great depth of 

 water. Perhaps the most notable instance of this kind is 

 afforded by Albemarle Sound. Where this estuary traverses 

 the Dismal Swamp, its main channels have, at low tide, only 

 four or five feet of depth, but the waterways which penetrate 



