Sinkhole risk in Calhoun County, SC: Sinkhole country

Sinkhole country

Soluble rock at or near the surface across much of the county.

On soluble rock: 80%
At or near the surface: 62%
Buried under cover: 18%

What to do about a house in Calhoun County

Get a sinkhole or geotechnical inspection before buying, building or adding weight to a foundation.

That works out to roughly 303 of Calhoun County's 381 square miles on soluble rock, 236 of them with the rock at or near the surface. With about 14,249 residents (37 per square mile), much of the land is open, and the mapped rock may sit far from where people actually live. The figures come from 419 map points about 1.0 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why the 18% under cover still counts, and the signs to look for and how an inspection works.

Counties around Calhoun County

The closest county with no soluble rock mapped is Fairfield County, SC, about 54 miles north. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Orangeburg County, SC 17 mi south · 72% at surface
Richland County, SC 25 mi north · 47% at surface
Sumter County, SC 28 mi northeast · 77% at surface
Lexington County, SC 32 mi northwest · 15% at surface
Clarendon County, SC 32 mi east · 79% at surface
Bamberg County, SC 36 mi southwest · 84% at surface

Which formation it is

The largest mapped soluble unit here is the Huber/Lisbon/Barnwell Formations, undivided (sand), Eocene in age, covering about 47% of the county. Next is the Duplin Formation at 24%.

Against the rest of South Carolina

Calhoun County ranks 14 of 46 in South Carolina for soluble rock at the surface, and sits above 90% of US counties on that measure.