Cape Fear Arch has vast influence

Why does the Lowcountry look the way it does? Why are there earthquakes? Why did rice grow so well and why were there phosphate beds rich enough to mine?

The easy answer — because it's a little rumpled.

The region is squished by a geological freak you've never heard of — the Cape Fear Arch. The arch is an uplift of the underground, running diagonally from the North Carolina mountains toward Cape Fear. It started rising millions of years ago and it's still coming up, a centimeter or two per year, lifting the land as far south as Cape Romain, just north of Charleston.

The arch acts like a wedge, pushing aside sediments that are deposited on top of it.

"It created the conditions for the rice culture. It created the conditions that were necessary to create those phosphate beds. This ancient arch has basically created our economic development the past few centuries," said Scott Harris, College of Charleston geology professor.

It did more than that.

The arch is tectonic, and that means seismic. Charleston, as it turns out, is in a "hinge zone" of the arch, the point where its push tends to meet the shove of other features underground. That creates what Robert Weems, of the U.S. Geological Survey, called the scissors-like compression of the Adams Run and Charleston seismic faults deep underneath the Ashley River.

And that's why the Lowcountry tends to have earthquakes, a few each year and a monster such as the 1886 quake every few hundred years or so.

Because of the arch, Cape Fear juts out to sea and there are bowls of relatively smooth coastline to its north and south. The bowl it forms along the South Carolina coast to Cape Romain means that Charleston tides tend to run 6-8 feet, while Myrtle Beach tides tend to run smaller.

Those tides and waves flood the Charleston coast into inlets and barrier islands. The bowls also tend to influence just where hurricanes make landfall.

The arch is why dozens of plant species found in the Waccamaw region along its tip are not found anywhere else. It's distinct enough that when environmental agencies straddling both states, including the Coastal Conservation League, formed a coalition they named it the Cape Fear Arch Conservation Collaboration. They point to its singular features to argue for preservation efforts.

"A lot of people haven't heard of the arch and we're going to change that," said collaboration coordinator Kristen Howell.