SinkholeOdds

Sinkhole risk in Duval County, FL: Pockets of it

Pockets of it

Soluble rock at the surface or under thin cover in part of the county.

On soluble rock
80%
At or near the surface
30%
Buried under cover
50%

What this means before you buy

Check the parcel against your state survey sinkhole map, and ask for an inspection if cracks, depressions or sticking doors show up.

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 612 of Duval County's 763 square miles on soluble rock, 230 of them with the rock at or near the surface. With about 1,055,159 residents (1,383 per square mile), most of the county is built up, so a mapped unit is likely to sit under somebody's street. The figures come from 258 map points about 1.7 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: what Florida insurers have to cover, and why gypsum and salt move faster than limestone.

Counties around Duval County

The nearest county rated sinkhole country is Nassau County, FL, about 20 miles north, close enough that the same rock may run under the county line. 3 of the 6 closest counties land in the same band; the ground changes quickly around here.

The rock underneath

The largest mapped soluble unit here is the Shelly sediments of Plio-Pleistocene age (beach sand), Holocene in age, covering about 50% of the county. Next is the Hawthorn Group, Coosawhatchie Formation, Charlton Member at 30%.

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

Duval County ranks 43 of 67 in Florida for soluble rock at the surface, and sits above 79% of US counties on that measure.