Louisiana is part of a sedimentary plain that slopes
gently toward the Gulf of Mexico. The tilted strata that constitute
this plain are pierced in the northwest and coastal area by enormous
plugs of salt called salt domes. The most pronounced relief in the
land surface is found in the northern hills and the area north of
Lake Pontchartrain. The flattest terrain is on the coastal marshes.
The dominant physical feature of the state is the Mississippi
Five natural regions are recognizable. The coastal marshes have either a firm surface or are soft, depending on the salt content. The gently sloping alluvial valley of the Mississippi lies toward the east, with its channels forming a bird-foot-shaped delta. The Red River valley, running northwest to southeast, follows the Mississippi's pattern on a much smaller scale. The terraces comprise the prairies in the southwest and the flatwoods to their north. The loessial bluffs flanking the Mississippi and containing moderate relief are also part of the terraces. The hill region, found in the northwest, is the oldest and highest part of the state.
The most important rivers in Louisiana are the MISSISSIPPI, RED, Atchafalaya, and Ouachita. Stream patterns are usually dendritic, resembling the branches of a tree. The Mississippi flood plain, however, is lower than the natural levees along the main watercourse, and water therefore drains away from the river. As a result, tributaries of the Mississippi in the state are insignificant. Much of Louisiana's terrain is related to the Mississippi.
Lakes, including an increasing number of artificial reservoirs such as Toledo Bend, are found throughout the state. Along the Mississippi and Red are a number of bayous or oxbows, formed as the rivers cut across their own meanders or when channels were cut off for flood control. False River and Raccourci Old River on the Mississippi are examples. Some larger lakes such as PONTCHARTRAIN and Maurepas are the result of subsurface faulting. In the west, shallow lagoons that formed behind beach ridges eventually created Sabine and Calcasieu lakes. To the east, subsidence of deltaic sediments formed round lakes. At the time of Spanish exploration, logjams had dammed the Red River and its tributaries, forming temporary raft lakes.
Louisiana's immature transported, or alluvial, soils comprise the most fertile land. They are associated with the Mississippi and Red River flood plains. The organic marsh soils are high in natural fertility but poorly drained and subject to flooding.
Residual soils of the uplands are derived from older sediments and are sandy and infertile. They are used primarily for grazing and forestland. The finely grained terrace soils of the prairies are older alluvial soils underlain by a claypan layer. This stratum of clay, coupled with low relief, causes slow drainage. Flatwoods soils are older and of low fertility.
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