SinkholeOdds

Sinkhole risk in Martin County, FL: Sinkhole country

Sinkhole country

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

On soluble rock
95%
At or near the surface
87%
Buried under cover
9%

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 519 of Martin County's 544 square miles on soluble rock, 471 of them with the rock at or near the surface. With about 165,666 residents (305 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 262 map points about 1.4 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 Martin County

The closest county with no soluble rock mapped is Gulf County, FL, about 353 miles northwest. 5 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Which formation it is

The largest mapped soluble unit here is the Shelly sediments of Plio-Pleistocene age (limestone), Pliocene/Pleistocene in age, covering about 55% of the county. Next is the Anastasia Formation at 32%.

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

Martin County ranks 8 of 67 in Florida for soluble rock at the surface, and sits above 96% of US counties on that measure.