the Mississippi River and forms a distinct physiographic unit bounded on the 

 west by the Chenier Plain, on the east and south by the Gulf of Mexico, and on 

 the north by a distinct contact with older, gulfward-dipping Pleistocene 

 deposits (Kolb and Van Lopik 1966). 



7. Progradation of the present and former Mississippi River deltas is 

 responsible for creating the recent deltaic plain of southwestern Louisiana. 

 Each time the Mississippi River has built a major delta lobe seaward, it has 

 subsequently been abandoned in favor of a shorter more direct route to the 

 sea. These course changes and accompanying shifts in centers of deposition 

 have resulted in the distribution of deltaic sediments along the coast of 

 southwest Louisiana. As soon as a delta lobe is abandoned, marine transgres- 

 sion caused by compaction of deltaic sediments begins. 



8. The geologic history of Louisiana's deltaic plain has been deter- 

 mined from more than 30,000 borings and hundreds of radiocarbon age determi- 

 nations. These data indicate that over the past several thousand years marked 

 changes have occurred in the Louisiana coastline. The evolution of Isles 

 Dernieres and similar delta-margin islands is closely related to these changes 

 in coastline. Because the Mississippi River has changed its course so often 

 in the last 8,000 years, the Louisiana coast offers an extremely dynamic area 

 in which to observe the various aspects of coastal sedimentation. Important 

 contributions to the understanding of the history of the Louisiana deltaic 

 plain have been made by Fisk (1955), Fisk and McFarlan (1955), McFarlan 

 (1961), Kolb and Van Lopik (1966), and Frazier (1967). 



9. During the last glacial advance, the Late Wisconsin Stage, conti- 

 nental ice accumulation caused sea level to be lowered some 90 m below its 

 present level (Dillon and Oldale 1978). As a result, the Louisiana shoreline 

 was as far as 1 60 km south of its present position (Kolb and Van Lopik 1958.) 

 Lowered sea level led to the entrenchment of gulfward-flowing streams and 

 their tributaries into the newly exposed deposits of the Pleistocene Prairie 

 Formation. The Prairie Formation is recognized in the subsurface by its 

 erosional contact, its light-colored-oxidized surface resulting from subaerial 

 exposure during lowered sea level and low water content. At that time, the 

 ancestral Mississippi River trended southeasterly across the coastal area 

 approximately 25 km west of Houma, Louisiana, and formed a valley approx- 

 imately 16 to 40 km wide (Kolb and Van Lopik 1966). 



10. Between 17,000 and 20,000 years B.P., sea level began to rise as a 



