NO. 10 MISSISSIPPI EMBAYMENT SHORE LINES COOKE 37 



In many places the shore at the 6- foot stage was so low that it 

 cannot be traced on maps with a 5-foot contour interval. An excep- 

 tion is at Ponchatoula, where a bluff rises about 10 feet above the 

 5-foot contour line, which bounds the marshes north of Lake Marepas. 

 This bluff continues westward into the Springfield, La., quadrangle. 



The shore of the Gulf during Silver Bluff time is marked by a 

 narrow barrier beach, rising 10 feet above the 5-foot line, that 

 curves across the southwestern part of the Lake Charles quadrangle. 

 Grand Lake School, in sec. 16, T. 12 S., R. 8 W., stands on it. 



RESUME OF EVENTS 



At the beginning of the story, presumably in Pliocene or early 

 Pleistocene time, the shore of the Gulf lay beyond the present sea- 

 shore, and rivers flowed across the Mississippi Embayment in steep- 

 walled valleys carved in bedrock. Then, perhaps during the first 

 interglaciation, the sea rose and flooded the embayment to a height 

 of 360 feet above its present level. 



This inundation produced a great bay extending about 250 miles 

 northeastward from Little Rock and averaging about 100 miles in 

 width. Later, the Gulf established shore lines successively near 275 

 feet, 215 feet, 170 feet, 145 feet, 100 feet, 70 feet, 42 feet, 25 feet, 

 and 6 feet. There were probably intermediate lower stands whose 

 locations have not been established. At each level the drowned valleys 

 formed successively smaller bays. 



In each bay the rivers deposited their load of sediment not far from 

 the shore, filling up the head of the bay and usually extending distribu- 

 taries of a delta into deeper water. These distributaries were aban- 

 doned when sea level fell to a lower level. At the present stage the 

 embayment is completely filled and the Recent delta bulges out into 

 the Gulf. 



Thus developed a series of terraces bordering a very gently sloping 

 area across which the once-rapid streams now meander. The old 

 shore lines within the central area are masked by veneers of alluvium 

 except where distributaries of Pleistocene deltas, their channels now 

 occupied by minor streams, stand above the general level. 



Objection will doubtless be raised to this theory of the growth of 

 the terraces and the alluvial plain of the Mississippi Embayment 

 because the sediments appear to be fluvial deposits. They have every 

 right to look like fluvial deposits, for they were brought down by 

 rivers, dropped for the most part as tidal flats and deltas in water that 



