The rolling green hills, flowing streams, and wooded peaks of the Yadkin Valley provide no obvious clues to the area’s ancient beginnings. However, the trained geologist sees plentiful signs that our peaceful and fertile Valley was born long ago in violent volcanic activity. It began about 540 million years ago in a chain of volcanic islands… 

Long before the North American continent attained its present form, two oceanic plates collided off the coast of the ancient African continent. One of the oceanic plates was forced deep into the earth beneath the other plate (called subduction). A combination of forces caused a portion of the upper oceanic plate to melt and form molten rock, or magma. The magma would eventually erupt onto the ocean floor, forming submarine volcanoes.  

Figure 1 from pubs.usgs.gov/publications/text

The submarine volcanoes that broke the ocean surface formed island volcanoes. Such volcanoes are typically strung out in chains called a volcanic island arc. (see fig 1.) 

Those volcanoes that were to eventually provide material to the northern piedmont area were part of a larger island arc named Carolinia (see fig. 2). 

This island arc first formed hundreds of miles from ancient North America off the coast of ancient Africa. In the movement of earth’s crust, the Carolinia arc is believed to have collided and fused itself onto ancient North America approximately 450 million years ago.     

When Carolinia collided with ancient North America the layers of
volcanic rock were changed due to heat and pressure (a process
known as metamorphism to geologists) and folded into great arches
causing the rock layers to be turned on edge. 

Approximately 300 million years ago the ancient continent of Africa also collided with ancient North America, forming the temporary supercontinent Pangea and the Appalachian mountain chain. Then about 245 million years ago, this supercontinent began to split apart, forming the Atlantic Ocean in the gulf between the pieces.  

Over a period of approximately 170 million years, through the Triassic and Cretaceous periods, the great mountain range that had been formed during the creation of angea was eroded¹ down to a gently undulating land surface.² Over millions of years, streams and rivers carried great deposits of Appalachian sand, silt and clay to the east.  These deposits gradually covered the land in layers, forming the Coastal Plain portion of the state. 

During the Cenozoic Era (66 Million years ago to the present), the Piedmont continued to erode, but at a slower pace. As the land surface was eroded away and the earth’s crust throughout the Piedmont became correspondingly lighter, the Piedmont was uplifted.³ 

Millions of years of erosion leveled the landscape to a nearly flat plain with gently meandering streams and rivers. Approximately 5 million to 1 million years ago, the rivers and streams of the Piedmont became entrenched in their floodplains. They cut down and incised their channels as the land surface slowly uplifted around them. The Yadkin River owes its origin to this period.

The erosion process continues to this day. The Yadkin still carries silt, sand, and clay deposits from its drainage area toward the coast.⁴

Geologically, Davidson County is divided into two main bodies of rock formations, each of which are characterized by a preponderance of related rock groups. (Such distinct bodies are known as “terranes”). Separating the two terranes is an extinct geological fault known as the Gold Hill fault zone. The rocks to the west of the Gold Hill fault zone are part of the Charlotte terrane; east of the fault, the rocks are part of the Carolina terrane, also known as the Carolina slate belt.  

While the rocks on both sides of the fault originated in a volcanic island arc, they are markedly different. The rocks of the Charlotte terrane are believed to have originated deep inside a volcanic arc while the Carolina terrane rocks are believed to have erupted or been otherwise deposited onto the surface of the volcanic islands.

Carolina terrane rocks in the Southeast portions of Davidson County exhibit features that are identical to modern-day volcanic island arcs. Here you’ll find two basic rock types: 1) volcanic rocks that were spewed out of erupting volcanoes and 2) sedimentary rocks (siltstones and mudstones) formed from the breakdown of those volcanic rocks.  

The volcanic rocks are typically very hard and resistant to erosion in comparison to the soft sedimentary siltstones and mudstones. Because of the folding that occurred when the volcanic island arc first collided with ancient North America, the rock formations of the Carolina terrane generally lie in a northeast – southwest orientation. 

This difference in rock hardness coupled with the northeast-southwest trend of the rock units greatly controls the landscape of the southeast portion of Davidson County. The harder volcanic rocks generally form the ridges and highlands with the siltstones and mudstones forming the lowlands and less hilly areas of that part of the county. For example, many of the creeks and rivers (Lick Creek and Flat Swamp Creek) (see Fig 3) follow the trend of the rocks and have eroded into the siltstones and mudstones.⁵  

In contrast, the high rocks under High Rock Lake dam, Flat Swamp Mountain,⁶ Grist Mountain, Wildcat Mountain and Three Hat Mountain in southeast Davidson County all form a prominent northeast trending ridge of volcanic rocks. These rocks record a period of violent volcanic eruptions that spewed lava, and hot avalanches of debris and ash. These volcanic deposits later formed a very dense and hard rock in comparison to surrounding softer siltstones and mudstones. 

The geologic processes that formed Davidson County and the Yadkin River Valley began hundreds of millions of years ago. They continue to this day, quietly changing the face of the land.  

Millions of years ago, plants and animals appeared and evolved. Some would become extinct. Man is a newcomer, appearing here about 12,000 years ago. If the 540 million year geologic history of Davidson County were reduced to one single 24-hour day, the period of man’s presence would extend a bit less than 1.33 seconds. The presence of people of European descent would take up only 1/20th of a second.

Copyright J. M. Daniel, 2006
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Notes: 

1.  The mountains were essentially broken down into sand, silt and clay and transported along streams and rivers to the present-day east and southeast, gradually covering the faulted continental margin. The sediment was deposited in layers starting in the Jurassic period and continues today. These sediments make up the Coastal Plain portion of the state. The Coastal Plain is an east-dipping wedge of sediment that is only a few feet thick just east of Raleigh but thickens to more than 10,000 feet below Cape Hatteras. 

2.  In geologic terms such a feature is known as a peneplain. 

3.  Geologists would refer to the forces at work here as Isostatic forces. Continental plates essentially float on top of the earth’s mantle. As rock is removed by erosion the earth’s crust will react by uplifting a proportional amount, similar to how a boat will “sit high in the water” when its cargo is removed. 

4.  The sand, silt and clay deposits present in the bed of the Yadkin River are slowly making a long journey toward the Atlantic Ocean. During flood events, as the water level rises, sand, silt and clay particles are transported over the riverbanks and are deposited. These deposits are known as alluvium or flood plain deposits. Alluvium is the youngest geologic unit within the Yadkin River. 

5.   Rock hardness also plays a role in the presence of Abbotts Creek. Abbotts creek occupies the Gold Hill fault zone. The ancient movement of the fault caused the rock to be weaker than surrounding rock. 

6.  Although the Uwharries carry the title of “Mountains” they are geologically not mountains at all but “Inselbergs,” which are peaks or outcroppings of hard stone left standing after the erosion of softer materials around them.