The Gulf Coastal Plain's southern boundary is the Gulf of Mexico in the U.S. and the
Sierra Madre de Chiapas in Mexico. On the north, it extends to the
Ouachita Highlands of the Interior Low Plateaus and the southern
Appalachian Mountains. Its northernmost extent is along the
Mississippi embayment (Mississippi Alluvial Valley) as far north as the southern tip of
Illinois. and south along the
Apalachicola River through the Florida panhandle. The flat to rolling topography is broken by many streams, river
riparian areas, and marsh wetlands.
United States section The Gulf Coastal Plain is a westward extension of the
Atlantic Coastal Plain around the
Gulf of Mexico. It is only the lower, seaward part of this region that deserves the name of plain, for there alone is the surface unbroken by hills or valleys. The inner part, initially a plain, has been maturely dissected into an elaborate complex of hills and valleys, usually of increasing altitude and relief as one passes inland. The Gulf Plain features not found in the Atlantic coastal plain are: • the
peninsular extension of the plain in
Florida • the belted arrangement of relief and
soils in
Alabama and in
Texas • the
Mississippi embayment or inland extension of the plain half-way up the course of the
Mississippi River to its junction with the
Ohio River at
Cairo, Illinois, with the Mississippi flood plain there included.
Florida peninsula A broad, low crustal arch extends southward at the junction of the Atlantic and Gulf coastal plains. The emerged half of the arch, constitutes the visible lowland peninsula of Florida. The submerged half extends westward under the shallow Florida overlapping waters of the Gulf of Mexico. The northern part of the peninsula is composed largely of a weak
limestone. Here, much of the lowland drainage is underground forming many
sinkholes (swallowholes). Many small lakes in the lowland appear to owe their basins to the solution of the limestones. Valuable
phosphate deposits occur in certain districts. The southern part of the state includes the
Everglades, a large area of low, flat, marshy land, overgrown with tall reedy grass. The eastern coast is fringed by long-stretching sand
reefs, enclosing
lagoons so narrow and continuous that they are popularly called rivers. At the southern end of the peninsula is a series of
coral islands, known as the
Florida Keys. They appear to be due to the forward growth of corals and other lime-secreting organisms towards the strong current of the
Gulf Stream from which they obtain their food. The western coast has fewer, shorter off-shore reefs. Much of it is of minutely irregular outline.
Alabama – Mississippi belted plain A typical example of a belted coastal plain is found in
Alabama and the adjacent part of
Mississippi. The plain is here about wide. The basal formation is chiefly a weak limestone, which has been stripped from its original Alabama innermost extension and worn down to a flat inner lowland of rich black soil, thus gaining the name of the black belt. The lowland is enclosed by an upland or
escarpment, known as
Chunnenuggee Ridge, sustained by partly consolidated sandy
strata. However, the upland is not continuous, but a maturely dissected escarpment. It has a relatively rapid descent toward the inner lowland, and a very gradual descent to the coast prairies, which become very low, flat and marshy before dipping under the Gulf waters, where they are generally fringed by off-shore reefs.
Mississippi embayment The coastal plain extends inland on the axis of the
Mississippi embayment. Its inner border affords admirable examples of topographical discordance where it sweeps northwestward square across the trend of the piedmont belt, the ridges and valleys, and the plateau of the
Appalachians. All of which are terminated by dipping gently beneath the unconformable cover of the coastal plain strata. In the same way the western side of the embayment, trending south and southwest, passes along the lower southeastern side of the dissected
Ozark plateau of southern
Missouri and northern and central
Arkansas. The southern Missouri and northern Arkansas Ozark plateau resembles in many ways the Appalachian plateau. The Ozarks and Ouachitas make up the
U.S. Interior Highlands, the only major mountainous region between the
Rocky Mountains and the
Appalachian Mountains. As the coastal plain turns westward toward
Texas, it borders the
Ouachita Mountains of Arkansas and
Oklahoma and the
Arbuckle Mountains in southern Oklahoma. The Ouachitas and the Arbuckles may be considered an analog of and possible extension of the Appalachian fold and crystalline belts.
Mississippi drainage basin In the embayment of the coastal plain some low escarpment-like belts of hills with associated strips of lowlands suggest the features of a belted coastal plain. The hilly belt or dissected escarpment determined by the Grand Gulf formation in western Mississippi is the most distinct. Important
salt deposits occur in the coastal plain strata near the coast. The most striking feature of the embayment is the broad valley which the Mississippi has eroded across it. The small proportion of total water volume supplied from the great
Missouri River basin is due to the light precipitation in that region. The lower Mississippi has no large tributaries from the lower east, but two important ones come from the west. The Mississippi Arkansas drainage area being a little less than the Ohio River and the basin of the
Red River of
Louisiana being about half as large. The Mississippi River drains an area of about one-third of the United States. The head of the coastal plain embayment is near the junction of the Ohio and the Mississippi. It flows southward for through the semi-consolidated strata of the plain. The river has eroded a valley about wide enclosed by bluffs one or two hundred feet high in the northern part. These bluffs decrease towards the south, but with local increase of height associated with a decrease in flood plain breadth on the eastern side where the Grand Gulf escarpment is traversed. This valley in the coastal plain, with the much narrower rock-walled valley of the upper river in the prairie states, is the true valley of the Mississippi River. However, in popular usage, the Mississippi valley is taken to include a large central part of the Mississippi
drainage basin. The valley floor is covered with a floodplain of fine silt, having a southward slope of only half a foot to a mile (100 mm/km). The length of the river itself, from the Ohio mouth to the Gulf is about due to its windings. Its mean fall is about 3 inches per mile (50 mm/km). On account of the rapid deposition of sediment near the main channel at times of overflow, the flood plain, as is normally the case on mature valley floors, has a lateral slope of as much as 5, 10, or even in the first mile from the river, but this soon decreases to a less amount. Thus just a short distance from the river, the flood plain is often swampy, unless its surface is there aggraded by the tributary streams. For this reason Louisiana, Arkansas and Mississippi rank immediately after Florida in swamp area. The great river receives an abundant load of silt from its tributaries, and takes up and lays down silt from its own bed and banks with every change of velocity. The swiftest current follows the outer side of every significant curve in the channel. Thus the concave bank on the side of the fastest part of the river is worn away. Any chance irregularity is exaggerated, and in time a series of large serpentines or meanders is developed, the most-symmetrical examples at present being those near
Greenville, Mississippi. The growth of the meanders tends to give the river continually increasing length. This tendency is counteracted by the sudden occurrence of cut-offs from time to time, so that a fairly constant length is maintained. The floods of the Mississippi usually occur in spring or summer. Owing to the great size of the drainage basin, it seldom happens that the three upper tributaries are simultaneously flooded. It is a serious problem for the lower river if two of the large tributaries flood at the same time. In this case, the lower river will rise to 30, 40 or even . The fall of the river is significantly steepened and its velocity is accelerated down stream from the point of highest rise. Conversely, the fall and the velocity are both diminished up stream from the same point. The load of silt carried down stream by the river finally, after many halts on the way, reaches the waters of the Gulf. There, the decrease of velocity aided by the salinity of the sea water, causes the formation of a remarkable delta, leaving less aggraded areas as shallow lakes (Lake Pontchartrain on the east, and Grand Lake on the west of the river). The ordinary triangular form of deltas, due to the smoothing of the delta front by sea action, is here wanting, because of the weakness of sea action in comparison with the strength of the current in each of the four distributaries or passes into which the river divides near its mouth.
Coastal plain in Louisiana and Texas After constriction from the Mississippi embayment to in western Louisiana, the coastal plain continues southwestward with this breadth until it narrows to about in southern
Texas near the crossing of the
Colorado River (Texas Colorado River, not the Colorado River that flows through the Grand Canyon), but it again widens to at the national boundary as a joint effect of embayment up the valley of the
Rio Grande and of the seaward advance of this river's rounded delta front. These several changes take place in a distance of about . It includes a region of over , less than half of the large state of Texas. A belted arrangement of reliefs and soils, resulting from differential erosion on strata of unlike composition and resistance, characterizes almost the entire area of the coastal plain. Most of the plain is treeless prairie, but the sandier belts are forested. Two of them are known as cross timbers, because their trend is transverse to the general course of the main consequent rivers. An inland extension from the coastal plain in north-central Texas leads to a large escarpment known as Grand Prairie (not structurally included in the coastal plain), upheld at altitudes of 1,200 or by a resistant Cretaceous limestone. This dips gently seaward with its scalloped inland-facing escarpment overlooking a denuded central prairie region of irregular structure and form. Its gentle coastward slope of per mile (3 m/km) is dissected by many branching consequent streams. In its southernpart as it approaches the Colorado river, the escarpment is dissected into a belt of discontinuous hills. The western cross timbers follow a sandy belt along the inner base of the ragged escarpment of Grand Prairie. The eastern cross timbers follow another sandy belt in the lowland between the eastern slope of Grand Prairie and the pale western escarpment of the immediately eastward and lower Black Prairie escarpment. This escarpment is supported at an altitude of or less by a chalk formation, which gives an infacing slope some in height. Its gently undulating or rolling seaward slope of 2 or 3 ft per mile (500 mm/km), covered with marly strata and rich black soil, determines an important cotton district. Then comes the East Texas timber belt, broad in the northeast, narrowing to a point before reaching the Rio Grande, a low and thoroughly dissected escarpment of sandy Eocene strata. This is followed by the Coast Prairie, a very young plain, with a seaward slope of less than 2 ft per mile (400 mm/km), its smooth surface interrupted only by the still more nearly level flood plains of the shallow, consequent river valleys. Near the Colorado river, the dissected escarpment of the Grand Prairie passes southward changing to a more nearly horizontal structure into the dissected Edwards plateau. The Edwards plateau is referred to later as part of the Great Plains. The plateau terminates in a maturely dissected fault scarp approximately 300 or in height as the northern boundary of the Rio Grande embayment. From the Colorado to the Rio Grande, the Black Prairie, the timber belt and the Coast Prairie merge in a vast plain, little differentiated, overgrown with chaparral (shrub-like trees, often thorny), widening eastward in the Rio Grande delta and extending southward into
Mexico. Although the Coast Prairie is a sea bottom of very modern uplift, it appears already to have suffered a slight movement of depression. Its small rivers all enter embayments. However, the larger rivers seem to have counteracted the encroachment of the sea on the land by a sufficiently active delta building with a resulting forward growth of the land into the sea. The Mississippi has already been mentioned as rapidly building forward its digitate
delta. The
Rio Grande, next in size, has built its delta about forward from the general coastline. Since this river is much smaller than the Mississippi, its delta front is rounded by seashore effects. In front of the
Brazos and the Colorado, the largest of the Texan rivers, the coast-line is very gently bowed forward as if by delta growth. The sea touches the mainland in a nearly straight shore line. Nearly all the rest of the coast is fringed by off-shore reefs or
barrier islands, built up by waves from the very shallow sea bottom. Due to the weak
tides, the barrier islands continue in long unbroken stretches between the few inlets. ==Habitats==