levees toward the interior of the islands, with finger-like extensions 

 penetrating several marsh types, and it terminates in open "inside ponds." 



The geologic development of the entire Mississippi River Delta is closely 

 associated with changing sea level over the past 50,000 yr, since the close 

 of the Peorian Interglacial stage. As sea level fell at the beginning of Late 

 Wisconsin glaciation, the Mississippi River became entrenched in the prairie 

 terrace, deposited during the Peorian. With this entrenchment, groundwater 

 table height fell. This resulted in oxidation, leaching, and weathering of 

 the soil on the exposed prairie surface (Fisk and McFarlan, 1955). 



As the Late Wisconsin glaciation decreased, raising the sea level toward 

 its present level, the river gradient and carrying capacity decreased. The 

 entrenched valley was filled by a sequence of sands and gravels under fine 

 sands, silts, and clays. In the late phase of sea level rise, over the last 

 5,000 yr, the Mississippi River prograded its delta over 11,395,900 ha (44,000 

 square mi) of the continental shelf and slope, depositing more than 33,345 km^ 

 (8,000 cubic mi^) of sediment (Fisk and McFarlan, 1955). Various major sub- 

 deltas were constructed, abandoned, and, to various degrees, destroyed through 

 this period of relative sea level stability. The modern birdsfoot delta lies 

 at the end of a 40-km (25-mi ) extension of the Mississippi deltaic plain. It 

 was initiated about 1500 A.D. (Fisk and McFarlan, 1955). 



The phases of delta development may be grouped as constructional, abandon- 

 ment, and destructional. The phases are the result of opposing rates of sediment 

 accumulation and relative subsidence, the latter representing both the effects 

 of subsurface compaction and sea level rise. In the constructional phase, depo- 

 sition exceeds subsidence, and the delta progrades. In abandonment, the two 

 processes approach t^ual rates. In the destructional phase, net subsidence 

 exceeds the rate of sediment accumulation, and marshland is lost to encroachment 

 of open water and to the action of wave and current erosion. 



The constructional phase begins as a stream discharges its sediment load 

 into a standing body of water. At this point, the stream mouth is unconfined 

 and its channel is wide and shallow. Channel-mouth sand bars are deposited, 

 which divert the flow of water to either side of the bar, beginning the 

 distributary network. As the distributaries elongate, the mouth bar progresses 

 gulfward forming long, finger-like sand deposits known as bar fingers. As the 

 channel further develops, natural levees form from flood-water deposition of 

 silty clay and clayey silt (Fisk and McFarlan, 1955). Sedimentary filling of 

 broad areas between the distributaries occurs frequently as flood water is not 

 yet confined by the levees. 



The growth of the branching channel network typically favors one path over 

 another because of differential stream gradients. The distributary with the 

 lesser gradient shoals rapidly and is abandoned. Eventually, even the main 

 distributary is overextended. A new channel is initiated by breaching the 

 levees. This decline in water and sediment inflow to the previous subdelta 

 area marks the beginning of the abandonment phase (Morgan, 1972). 



69 



