Sinkhole risk in Perry County, MO: Sinkhole country

Sinkhole country

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

On soluble rock: 89%
At or near the surface: 86%
Buried under cover: 2%

The next step for one property

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

That works out to roughly 420 of Perry County's 474 square miles on soluble rock, 409 of them with the rock at or near the surface. With about 19,100 residents (40 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 574 map points about 0.9 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: why gypsum and salt move faster than limestone, and the signs to look for and how an inspection works.

Counties around Perry County

The closest county with no soluble rock mapped is Perry County, IL, about 35 miles northeast. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Ste. Genevieve County, MO 23 mi northwest · 76% at surface
Cape Girardeau County, MO 23 mi south · 81% at surface
Jackson County, IL 24 mi east · under 1% at surface
Randolph County, IL 24 mi north · under 1% at surface
Bollinger County, MO 30 mi southwest · 91% at surface
Madison County, MO 34 mi southwest · 67% at surface

What Perry County is built on

The largest mapped soluble unit here is the Decorah Group, Plattin Group (limestone), Middle Ordovician-Mohawkian Series in age, covering about 30% of the county. Next is the Smithville Dolomite, Powell Dolomite, Cotter Dolomite, Jefferson City Dolomite at 18%.

The county also lies over the Illinois Basin, 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 Missouri

Perry County ranks 42 of 115 in Missouri for soluble rock at the surface, and sits above 96% of US counties on that measure.