Sinkhole risk in Florence County, SC: Sinkhole country

Sinkhole country

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

On soluble rock: 84%
At or near the surface: 77%
Buried under cover: 7%

What to do about a house in Florence County

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

That works out to roughly 672 of Florence County's 800 square miles on soluble rock, 619 of them with the rock at or near the surface. With about 138,049 residents (172 per square mile), homes are spread across towns and farmland, and a mapped unit may or may not reach the developed parts. The figures come from 534 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why the 7% under cover still counts, and the signs to look for and how an inspection works.

Counties around Florence County

The closest county with no soluble rock mapped is Union County, NC, about 81 miles northwest. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Marion County, SC 21 mi east · 33% at surface
Darlington County, SC 25 mi northwest · 87% at surface
Williamsburg County, SC 28 mi south · 31% at surface
Dillon County, SC 31 mi northeast · 79% at surface
Lee County, SC 32 mi west · 88% at surface
Clarendon County, SC 38 mi southwest · 79% at surface

The rock underneath

The largest mapped soluble unit here is the Waccamaw Formation (sand), Pleistocene in age, covering about 38% of the county. Next is the Bear Bluff Formation at 25%.

Against the rest of South Carolina

Florence County ranks 6 of 46 in South Carolina for soluble rock at the surface, and sits above 94% of US counties on that measure.