What Caused The Monolithic Rock Formations In South Carolina

Monadnocks occur along the border between the Carolinas.

The plains of the North Carolina-South Carolina border are dotted with massive peaks that seem out of place on the otherwise flat plateau. Although they stand solitary now, they were once part of extensive geological formations that stretched across the region. These kinds of monolithic formations also occur in North Carolina, New Hampshire and Georgia.

Monadnocks

Different kinds of rocks erode at different rates.

South Carolina’s monolithic rock formations, including Little Mountain, Caesar’s Head and Table Rock, are geological features called monadnocks. Monadnocks are mountains that were originally part of ranges, but over the centuries the rest of the mountains in the ranges eroded, leaving only the monolithic monadnocks standing. They always form in temperate areas with high humidity. These formations remain standing while the surrounding mountains erode because of the nature and composition of their rock.

Composition

Granite is very slow to erode.

Monadnocks are made of types of rock that are much slower to erode than the rocks around them. The monolithic formations in South Carolina are made of quartzite, or sometimes of granite or other dense volcanic rocks. When the Appalachian mountains were formed, the heat released by the layers of the Earth’s crust grinding against one another melted the rocks and pushed them to the surface. These layers of once-molten rocks became the monadnocks.

First Stage of Formation

The continents were originally one big landmass called Pangaea.

The monadnocks began to form before the continents split apart. Three hundred million years ago, the area of Pangaea (the original, single continent) that would become the coasts of Africa and North America began to shift and push against itself, and the rocks along that fault line began to melt. The molten rock shifted upwards and then cooled and solidified, forming hills and domes of hard rock within the layers of softer surrounding rock. This set the stage for the unusual formations that would develop over the millennia of erosion.

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Second Stage of Formation

The monadnocks have layers have different kinds of granite or quartzite.

When Pangaea began to break apart, roughly 200 million years ago, the forces acting on the Earth’s crust reversed. Instead of compressing together and melting due to the heat from that process, the rocks began to fracture and pull away from each other. During this process, molten rock from deeper inside the Earth bubbled up to the surface, where it cooled and hardened. This rock had a different structure from the first kind of molten rock, making a total of three erosion rates in the area — the original crust, which erodes fastest, and then two kinds of harder rock. These two both erode slowly, but at different rates from one another, forming the varied stratifications of the monadnocks.