Sinkhole risk in Nassau County, FL: Sinkhole country

Sinkhole country

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

On soluble rock: 87%
At or near the surface: 67%
Buried under cover: 19%

The next step for one property

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

Standard Florida homeowners policies cover only catastrophic ground cover collapse; full sinkhole coverage is a separately priced option insurers must offer.

That works out to roughly 563 of Nassau County's 649 square miles on soluble rock, 437 of them with the rock at or near the surface. With about 104,376 residents (161 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 485 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Nassau County

The closest county with no soluble rock mapped is Schley County, GA, about 189 miles northwest. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

Duval County, FL 20 mi south · 30% at surface
Camden County, GA 22 mi north · under 1% at surface
Charlton County, GA 25 mi northwest · 1% at surface
Baker County, FL 37 mi southwest · 21% at surface
Brantley County, GA 43 mi north · under 1% at surface
Clay County, FL 43 mi south · 51% at surface

The rock underneath

The largest mapped soluble unit here is the Hawthorn Group, Coosawhatchie Formation, Charlton Member (dolostone (dolomite)), Miocene/Pliocene in age, covering about 67% of the county. Next is the Shelly sediments of Plio-Pleistocene age at 19%.

The county also lies over the Floridan shelf, a deep salt or gypsum basin. Deep beds rarely cause surface collapse on their own, but wells and mining into them have.

Against the rest of Florida

Nassau County ranks 17 of 67 in Florida for soluble rock at the surface, and sits above 92% of US counties on that measure.