Effects of Glaciation
Pollen samples from many bog sites across the southeast, including east-central Texas, and fossil plants from the Tunica Hills of Louisiana and Mississippi, have allowed the reconstruction of climate and plant distribution in the West Gulf Coastal Plain. During the ice age, many plant species shifted southward, and at glacial maximum around 18,000 years ago, boreal forest reached Arkansas, with spruce and tamarack in the Tunica Hills along the cold, foggy Mississippi valley. Deciduous forests of oak and hickory extended to the Gulf of Mexico. As the glaciers retreated and the climate warmed, pines and other species from refugia in Florida, along the lower Mississippi River, and southern Texas and northern Mexico, spread across the southeast. The megafauna died off or were hunted to extinction by palaeoindians.
As glaciers retreated northward, the accumulation of water as glacial ice that had resulted in sea levels 100 to 120 meters below modern levels and a coastline that extended 100 to 200 km south, melted and flooded the coastal shelf, with about 20% of the coastal plain covered by rising waters. River grades lessened, and river flow slowed and began to meander.
The Postglacial Landscape
Today, boreal forests are found south only to the Great Lakes. The deciduous forests that once reached the Gulf now extend only as far south as the Missouri River. The mixed oak-pine-hickory forest and monospecific longleaf pine forest that now cover large parts of the southeastern United States are relatively new. The postglacial landscape may have been open oak-hickory woodlands and savannas with a rich herbaceous flora that extended over broad areas of the West Gulf Coastal Plain. Today, these are now confined to especially xeric habitats, notably in the western portion of the West Gulf Coastal Plain. By about 6000 years ago, the present forest composition had probably been attained over most of the West Gulf Coastal Plain.
