Coastal Plains Prehistory Part 1 of 3

Disclaimer: Most of this passage was derived from Fred Beyer’s: North Carolina the Years Before Man: A Geologic History. This post is a work in progress, please contact me if any of the information is incorrect.

Coastal Plain Prehistory: Part 1 of 3

What is the Coastal Plain?: A Brief Geological History

From an early age, students in North Carolina are taught to distinguish the three regions of North Carolina. These are the mountains, piedmont, and coastal plain. One can surmise how the mountain region got its name, but the other two are not as straightforward. The word piedmont is derived from the Italian word “piemonte”, meaning “mountain foot.” Considering the etymological origins of the piedmont region, it becomes clearer what constitutes this area. That is, a gradual tapering in elevation associated with foothills. Then there’s the coastal plain, relatively flat and featureless. But how did it get that way? Unlike the relative stability the mountains and piedmont have enjoyed over the last 100 million years, the coastal plain is a battered landscape molded by the unassailable power of water. To grasp the dramatic change this region has undergone we must begin at the dawn of the modern coastal plain, around 150 million years ago, at the beginning of the Cretaceous period.

Early-Mid Cretaceous (145 million-100 million B.P.)

The first 50 million years of the Cretaceous were marked by extensive erosion from rain and wind. Smoothing the ancient blue ridge mountains and muting the once jagged hills of the piedmont into a peneplain (meaning almost a plain). The eroded sediments flowed east creating large deposits of loose sediments across the coastal plain. By the end of the Early Cretaceous, the western edge of the Atlantic ocean was located just east of the coastal plain/piedmont border. 

The crust began to subside under the coastal plain, dropping the elevation of the landscape, thus moving the ocean further inland. Rivers and streams flowing into a rising ocean created new deltas, blanketing the cape fear formation (geological formation associated with dinosaur fossils) under a layer of sediment. The western edge of the newly formed layer of sediment stretched from present-day Martin, Bertie, and Hertford county(s) to South Carolina. This new layer of deposition is referred to as the Middendorf formation. 

Mid-Late Cretaceous (95 million-66 million B.P.)

Over the next 17-18 million years the water at the edge of the primordial coastline was very shallow. The dominant form the landscape took during this time was one of the myriad swamps and shallow, meandering streams that flowed eastward to an ever-shifting coastline. Fine sand, gray and black shales, and petrified wood were deposited during this time. The fossil-rich Black Creek Formation (BCF) took shape during this period, covering most of the southeastern portion of the coastal plain. Sediment analysis from the BCF suggests the presence of a large delta that supported a diverse range of life. 

By the end of the Cretaceous, the ocean receded from the coastal plain, leaving an erosional surface, a surface formed strictly by sediment deposition. As the water withdrew from the landscape, streams carved a path eastward through what was essentially a blank canvas of loosely packed sediment. At this point (66 million B.P.), a mass extinction wiped out most of the large dinosaurs. 

Paleocene (66 million-56 million B.P.)

By now, most of the coastal plain was above sea level. The meandering streams of the peneplain piedmont responded to the dropping sea level by cutting deeper into the earth. This caused the route of the older stream beds to become fixed by an ever-deepening channel. Little else is known about what took place in the Paleocene. Mostly due to the vast scale of erosion that occurred during the period.

 Eocene (55 million-36 million B.P.)

During the first 10 million years of the Eocene, erosion occurred throughout the state. Around 47 million years ago, volcanic eruptions were thought to have taken place, although erosion destroyed all evidence of the volcano’s presence. Around the same time, the ocean once again inundated the landscape. This time reaching new highs, flooding into the piedmont region. The volcanic debris deposited along the erosional surface was extremely nutrient-dense. Deepwater undisturbed by wave action set the stage for an explosion of life towards the present-day coast. The semi-tropical climate during this time spurred the growth of calcerous green algae. As each successional generation sank from the ocean’s surface, they deposited very fine grains of lime into a muddy seafloor, enabling invertebrates to build in great numbers. After 6 million years of this process, thick layers of lime and mud consolidated into limestone. Today this limestone is mined in Wilmington, New Bern, and Rose Hill.

Oligocene-Pliocene (35 million-1.8 million B.P.)

After the nutrient-rich coastal waters receded, an erosional event lasted until 28 million years ago. During this period, North Carolina had a subtropical climate. At this time, the modern “fall line” began to take shape. Around 20 million years ago a unique set of conditions formed in modern-day Beaufort county. The crust located under the coastal plain formed a hinge-like fault dividing the seafloor into two distinct formations. The eastern edge of the fault line sloped gently downward to deeper water, whereas the landward side was flat and subsiding. This combination created a basin in which cool ocean currents from the deep fed directly into. The cold water acted as a conveyor belt, delivering nutrient-rich water into a relatively shallow basin, resulting in an explosion of single-celled life. As the cold water met the warmer water inside the bay, phosphate precipitated onto the ocean floor along with the decaying organisms. As this process repeated over long periods, a giant layer of phosphate formed. The phosphate formed in Beaufort county is still mined in Aurora. 

The ocean rose and fell several times in the intervening years before the Pliocene. By the start of the Pliocene, the mountain and piedmont regions took a form that closely resembles their appearance today. Around 5 million years ago, the ocean moved inland, covering the eastern half of the coastal plain. With depths that ranged from 49-98ft. This time the formation of barrier islands accompanied the flooding. This created an inland lagoon that resembled the present-day sounds of eastern NC. The climate during this period was arid-temperate. The sediments deposited in this large lagoon formed the Yorktown formation. World-renowned for its high concentration of fossils, the Yorktown formation covers most of the northern coastal plain, just east of the modern towns of Roanoke Rapids, Rocky Mount, and Wilson. The Duplin formation was formed in the southern portion of the coastal plain. Although the Yorktown and Duplin formation share many characteristics, the distinctive fossil record of each suggests they were geographically isolated from one another.  Deposition onto the Yorktown and Duplin formation continued until about three million years when the land of the coastal plain rose, once again pushing the ocean westward. For the next 500,000 years, the Yorktown formation was shaped by erosion, where stream channels cut deeply into the sediments. It is thought that the deposits of the Yorktown formation formed Ocracoke and Hatteras Islands. 

Pleistocene (1.6 million-18,000 B.P.)

The Pleistocene coastal plain was marred by flooding caused by ice caps melting. The first of which is referred to as the Nebraskan Glaciation, occurring around 2.1 million years ago.  The periods of freezing and thawing are almost entirely due to fluctuations in the earth’s orbit. Most think of the earth as maintaining a stable orbit as it journeys around the sun. In reality, the earth is prone to small “wobbles”, changing its orientation to the sun’s radiation. Small changes in the amount of solar radiation reaching earth have giant consequences for the global climate. These variations in are referred to as the Milankovich cycle. In periods where solar radiation is less intense, ice caps grow in the earth’s poles, and vice versa. 

Before the Pleistocene, there was a gargantuan flooding event, even by geological standards. The polar ice completely melted, lifting the ocean over 300ft above its current levels. This flooding event caused escarpments (a slope or cliff formed by wave action at a former coastline) to take shape on the western edge of the coastal plain. Later in the Pleistocene, varying degrees of icecap-related flooding left several other escarpments still visible throughout the western coastal plain. Although the extent of the ice sheets never reached North Carolina, the climate was much colder than today, resembling that of modern-day Maine.  

End of Ice Age-Younger Dryas Period-Present (18,000-11,700-Present)

The melt and freeze of the polar ice continued to shape the land until the last glacial maximum (LGM) around 18,000 years ago. The period of warming that began at the end of the LGM slowly brought the ocean up towards its current level. At the peak of the LGM, sea levels were about 400ft lower than present, with the coastline sitting at the edge of the continental shelf. 

Between 50,000-6,000 years ago, a geological oddity took shape throughout the Southeastern U.S. Mysterious elliptical-shaped shallow depressions called Carolina Bays littered the coastal plain. There is no scientific consensus on the origins of the Carolina Bays. Strangely, nearly all of them are oriented northwest-southeast. Still, the most widely-accepted theory posits that prevailing southwest winds caused current in the water-filled bays to form their characteristic elliptical shape. Scientists asserted at least 14 other theories to explain this geological phenomenon. These include artesian springs, solution depressions, gyroscopic eddies, remnants of spawning beds left behind by giant prehistoric aquatic creatures, and most prominently, meteor impacts. The impact theory is supported by traces of iridium (an element found in extra-terrestrial rock) and magnetic anomalies around some Carolina bays. Sediment analysis taken from the bays suggests that the impact theory is unlikely. 

Around 12,900 B.P., an abrupt change in the climate swept through the northern hemisphere, causing a mini-ice age. This abrupt change is referred to as the Younger-Dryas period. The melting ice suddenly discharged massive amounts of water into the ocean. The influx of freshwater halted the thermohaline circulation (ocean currents responsible for temperature regulation and freshwater transport), creating a dramatic stratification of temperature. Above 40 degrees north (around central Pennsylvania) experienced arid, arctic-like conditions, sending northern latitudes back into an ice age. Due to the warm tropical water stalling off the southeastern coast, Caribbean air engulfed the areas below 40 degrees north, causing a warm wet climate. This event lasted for 1,200 years before the thermohaline circulation resumed the warming process started at the end of the LGM.

The last 10,000 years have been relatively quiet, in a geological sense. Casual observers can appreciate the shifting sands of the outer banks or the remnants of thousand-year floods in any of the state’s alluvial floodplains. But the coastal plain remains a dynamic landscape. The same processes that shaped this landscape are still at work. Humans can only slightly delay the momentum of geological and climatological events. We must keep in mind that the earth and its ancient forces still maintain their domain over the land. Working with and adapting to the earth’s natural rhythm will aid in our future survival. If we resist, we will be yet another species in the fossil record that could not cope with the ever-changing spherical rock we call home.