Natural History of North Carolina

Natural History of North Carolina

by Roger Shew

North Carolina has a long geological history, with the western part of the state having rocks greater than 1 billion years old. The state is divided into three physiographic provinces: from west to east, these are the Blue Ridge, Piedmont and Coastal Plain. The Blue Ridge comprises 10% of the state while the Piedmont and Coastal Plain each cover roughly 45% of the state. North Carolina becomes “younger” as you move east with active sediment movement and deposition in our rivers and on the coast. And with these geological changes from west to east there are also changes in the topography, resources and ecological diversity.

Roger Shew shares information about two limestone rocks mined from the Martin Marietta Quarry at Castle Hayne. The larger one, on the right, is classic Castle Hayne Limestone with lots of fossils and high porosity. The smaller boulder represents an unconformity, missing time in the geologic record, where 40 million-year-old Eocene Castle Hayne Limestone overlies 70 million-year-old Cretaceous age limestone rocks. This is a classic piece of geological history. Note the milkweed blooming in the rain garden behind Shew.

photo by Valerie Robertson

The Earth’s surface of continents and ocean basins was much different in the past compared to what we see today. These continents and ocean basins, termed “plates,” are in constant motion. Plate movement is described by the Theory of Plate Tectonics, the underlying theory in Geology.

The Blue Ridge and Piedmont regions were formed by multiple plate collisions, known as convergent plate boundaries. The buckling and subduction of the plate edges led to the formation of igneous and metamorphic rocks through the deep heating and melting of rock and the pressure and heat of the boundary collisions, respectively. The igneous rocks, or fire rocks, include ancient volcanoes and deep magmas that solidified into hydrothermal veins with gold and granitic batholiths/bodies of rock. These granitic bodies, which are now exposed at the surface because of weathering and erosion of the overlying rock, supply rich resources for North Carolina including feldspar, quartz and even gold. In fact, North Carolina was the original gold rush state in 1799 and the leading producer of gold until 1849.

During the convergent episodes of continental collisions, sediments and rocks were metamorphosed. (“Meta” means “change” and “morph” refers to “shape” or “form.”) The Piedmont is mostly composed of metamorphic rocks. You might notice from the geologic map below that the colors are mostly oriented NE to SW. This was the angle of continental collisions of island arcs and the Continent of Africa with North America (Africa was part of a much larger continent of Gondwanaland and North America was part of Laurasia).

The Blue Ridge and Piedmont were formed in the Proterozoic through the late Paleozoic. Although plates move at slow rates and mountains build and erode at slow rates, the long geologic time of over ½ billion years is adequate to move mountains and continents and to wear them down. The resultant mountains in North Carolina were once higher than 20,000 feet, but Mother Nature has relentlessly worn them away over hundreds of millions of years to the subtle, rounded mountains we know today. We do still have the highest mountain peak east of the Mississippi River: Mount Mitchell stands at 6,684 feet.

North America and Africa were joined together at the start of the Mesozoic. But the continents are in constant motion and another phase of continental movement began. North and South America started to move away from Africa and Europe along rift basins formed at a divergent boundary. A divergent boundary is a spreading center. North Carolina was once joined to the African countries of Mauritania and Senegal. Rifting continued with some basins finally opening to form the ancestral Atlantic Ocean.

Some of the rift basins ceased opening and formed small NE to SW trending sedimentary-filled basins. These are the Triassic Basins of North Carolina that include the Durham, Sanford and Dan River Basins. The sedimentary rocks include many red beds (source of clay/sand for bricks) of lake and river deposits as well as minor coal deposits, which were important in the Civil War. Numerous red beds are easily seen west of Sanford and at the RDU airport on the west side of Raleigh; this is the Durham Basin.

Following this rifting phase, there have been no major tectonic episodes in North Carolina. The last ~100 million years has been a time of weathering and erosion of the mountains with sediments brought to the coast to form rivers, deltas and beaches. Marine limestones and sandstones were forming offshore during this time. These eroded sediments and offshore deposits formed the Coastal Plain. The Coastal Plain is dominated by rocks that range from the Cretaceous to sediments moving today in our rivers and on our beaches.

Several important rock resources were formed in the shallow marine waters in this interval of time, including the Castle Hayne Limestone (40 million years ago); it is a significant source of aggregate and a very good aquifer. Another important Coastal Plain resource occurs near Aurora where phosphate from the Pliocene and Miocene (2–15 million years ago) is mined. The quarry is even better known for the presence of excellent marine fossils including whale vertebra, mollusks, gastropods and the large Carcharodon megalodon teeth.

The fossil pit in the GeoGarden contains a variety of the typical rocks and fossils from the Aurora Quarry. The rocks are mostly limestones and sandy limestones, but there are also samples of the black phosphate rocks that are mined as a resource. The fossils include coral, mollusk shells, mammal vertebrae and a variety of shark teeth. Digging through the fossil pit may yield a quality fossil for someone to take home. See article on the GeoGarden on page 9 of this issue.

photo by Valerie Robertson

One of the very important parts of the history of the Coastal Plain is the ever-changing shoreline. Sea level rise and fall have helped to form and modify the Coastal Plain Province over time. Numerous ancient shorelines are visible on the Coastal Plain on topographic and remote sensing maps. In fact, just 18,000 years ago, sea level was lower by ~400 feet during the last maximum glaciation event. You would have had to drive east 50 miles or so to be “at the beach.” Sea level has risen since that time, and our current shoreline with its extensive barrier islands was in place several thousand years ago. Sea level is rising today, and along with storms, continuing to modify our beaches.

The changes in elevation, soils and rocks, and even differences in the weather have led to the physiographic provinces having unique flora and fauna. This is another story to tell.

Roger Shew is in the Earth and Ocean Sciences and Environmental Sciences Departments at UNCW.