The Coastal Plain
Layers of young (geologically that is) rocks that tilt ever so slightly to the south at slopes of about 40 feet per mile occupy most of central and southern Alabama. The whole package of rocks can be likened to a tilted deck of cards with each card slipped slightly to the south from the one beneath it. This means that the most northerly rocks are the oldest and those at the coast are the youngest. The rocks are Mesozoic and Cenozoic in age and represent about 100 million years of geologic time.
Table 1 lists the geologic units in the Coastal Plain and their composition.
The Coastal Plain developed on geologically-young (Mesozoic to Recent: 138 millions of years to present) materials, most of which is sediment (unconsolidated materials, the precursors to sedimentary rocks). Thus, the geologic units are described variously as gravels, sands, silts and clays. Consolidated sediment in the form of chalk, sandstone, limestone and claystone is interbedded with the unconsolidated materials.
Figure CPG1. geologic Map of the Coastal Plain. (Alabama Geological Survey)
The beds slope (dip) gently at about 40’/mile and are progressively younger from the Fall Line towards the coast. In some cases they also thicken towards the coast.
The boundary between the oldest of these rocks and the youngest of the Paleozoic rocks, often called the Fall Line, represents about 150 million years of missing geologic history. Geologic boundaries that represent great periods of missing time are called unconformities.
Another major unconformity occurs within these rocks at the boundary between the Cretaceous and Tertiary rocks. During the time represented by this unconformity (often called Cretaceous-Tertiary boundary or K-T boundary) one of the great extinction in the history of the Earth took place. It is estimated that about 75% of the living species in the Cretaceous were wiped out in an extraordinarily short period of time. The most satisfactory explanation for this extinction is that the earth was struck with a huge meteorite, which set off a series of environmental disasters. Recently, geologists have identified a huge crater in the Yucatán peninsula of Mexico that could be the result of this meteorite. In Alabama, the evidence for this impact can be seen on the banks of the Tombigbee River, where churned up rocks of Cretaceous age occur.