NO. 10 MISSISSIPPI EMBAYMENT SHORE LINES — COOKE 5 



(Cooke, 1932, 1935; MacNeil, 1950; and others). It is hoped that 

 investigations now in progress in South Carolina (Colquhoun, 1962, 

 1964) will yield evidence that may point to more definite conclusions. 



THE TRACING OF EMERGED SHORE LINES 



Each lowering of sea level, whether caused by expansion of the ice 

 caps or by downwarp of the sea bottom, laid bare a new strip of 

 land; each melting of the ice caps raised the level of the sea and 

 drowned all valleys to the same height. During high stages bayhead 

 deltas accumulated in the drowned valleys; during lower stages 

 trenches were cut by the rejuvenated streams. 



The terrace laid bare by the retreat of the sea to a lower stand 

 is not level. It ranges in height from the altitude of the original 

 shore to that of the new. Moreover, it retains all the humps, hollows, 

 and slopes of the original sea bottom except as they may have been 

 modified by the retreat of the sea across them. 



The most characteristic feature by which one marine terrace can be 

 distinguished from another is the contemporary shore line. Even that 

 line is not completely level, for it is the mark made on the land by the 

 water, and the location of that mark varies with the force of the 

 waves and the height of the tides. Moreover, the depth of the water 

 at the shore varies from place to place and makes more difficult the 

 recognition of the location of the original surface. 



The tracing of an emerged shore line is further complicated by the 

 dissection that it may have suffered since the sea withdrew. In general, 

 the higher, older terraces have suffered more erosion, and their shore 

 lines are the most difficult to trace. However, the relief of the land 

 has more influence on the rate of erosion than its age. A high, wide 

 terrace may be better preserved than a lower, narrower one having 

 greater local relief. 



The altitude of an emerged shore line can generally be determined 

 with more precision within an estuary than along the open sea coast, 

 for tidal flats there give a very close approximation to the former 

 sea level. But tidal flats may be restricted to the head of the estuary ; 

 farther down the drowned valley the sides may slope steeply into 

 deep water, perhaps even form a vertical bluff. At such places high 

 water may leave little or no mark, or the narrow terrace may later 

 be eroded. 



The altitudes assigned to the shore lines in this paper are not 

 precise, but they probably do not differ from the average altitude 

 by more than a few feet, perhaps less than five feet. 



