Mesozoic Period
The most intense mountain building episode in the Appalachians took place in the Permian Period at the end of the Paleozoic, as Africa crashed into North America. With the end of the Paleozoic, such mountain-building in the Appalachians came to an end. At that time the mountains were high, steep, and very likely glaciated. As uplift rates decreased, however, the mountains swiftly began to erode away, so that low mountains replaced the alpine peaks. Crustal forces were not dead, however. In the Triassic and Jurassic Periods of the Mesozoic Era, North America and Africa began to pull apart. The modern Atlantic Oceanwas born. Itcontinues to expand even today as North America and Africa drift farther apart. This drifting produced many rift basins, mainly in the Piedmont. Crustal uplift also continued irregularly during the Mesozoic and Cenozoic, but of a much different type. Instead of a lateral compression producing folding and thrust faulting as well as uplift, a broad, gentle, vertical uplift predominated. The rate of uplift was also much lower than during the Paleozoic.
Changes in stream drainage during the Mesozoic
A major change in stream drainage also took place in the Mesozoic. Today, many streams in the Southern Appalachians drain to the Atlantic Ocean. When the high Paleozoic Appalachians dominated the topography, however, most streams flowed to the northwest. We know this because the flow-direction indicators found in Paleozoic rocks in the Ridge and Valley and Appalachian Plateau Provinces show that the sediments that make up these rocks were laid down by currents flowing in that direction. The question arises of how the direction of stream flow reversed after the end of the late Paleozoic mountain building.
Geologists and geographers have tried to answer this question for a century. The key to solving this problem comes from our knowledge of plate tectonics, and probably involves the pulling apart of North America and Africa that took place during the Mesozoic. We now know that when plates move apart, the crust near their margins usually subsides. This sinking may explain how formerly west-flowing streams reversed their direction of flow. Before the subsidence of the eastern margin of North America, the height of land that formed the southeast-northwest divide probably lay along the crest of the Paleozoic Appalachians near the present-day coastline. As the continental margin continued to sink, however, the divide moved farther and farther to the northwest. The northwestern part of the Appalachian region, which originally was lower, gradually became higher than the southeastern part. This southeastern slope made it possible for streams in the Appalachians to flow to the Atlantic (Judson, 1975).
Another event that took place during the later Mesozoic was the submergence of the Atlantic and Gulf continental margins. A look at a geologic map of the Southeastern United States shows that the southwestward trend of the Appalachians is truncated by Cretaceous sediments in western Alabama. Buried Appalachian structures continue beneath the Gulf Coastal Plain sediments as far as Oklahoma. The Cretaceous sediments also extend long distances inland from the present coastline from Alabama to North Carolina. Furthermore, it is very likely that the boundary between the Piedmont and the Coastal Plain (the Fall Line, so named because of the numerous waterfalls that occur along this boundary) has retreated a great distance seaward from its original position, and that the inland incursion of the Cretaceous seas was much greater than marked by todays Fall Line.
Encyclopedia ID: p1542
