Sinkhole risk in Washington County, MO: Sinkhole country

Sinkhole country

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

On soluble rock: 96%
At or near the surface: 96%
Buried under cover: 0%

What this means before you buy

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

That works out to roughly 732 of Washington County's 760 square miles on soluble rock, 732 of them with the rock at or near the surface. With about 23,583 residents (31 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 380 map points about 1.4 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: the signs to look for and how an inspection works, and how these percentages are measured.

Counties around Washington County

The closest county with no soluble rock mapped is Perry County, IL, about 84 miles east. 6 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Crawford County, MO 23 mi west · 100% at surface
Iron County, MO 24 mi southeast · 79% at surface
St. Francois County, MO 25 mi east · 71% at surface
Jefferson County, MO 29 mi northeast · 98% at surface
Franklin County, MO 34 mi north · 96% at surface
Dent County, MO 40 mi southwest · 100% at surface

What Washington County is built on

The largest mapped soluble unit here is the Eminence Dolomite, Potosi Dolomite (dolostone (dolomite)), Late Cambrian-Croixian Series in age, covering about 50% of the county. Next is the Gasconade Dolomite at 30%.

Against the rest of Missouri

Washington County ranks 31 of 115 in Missouri for soluble rock at the surface, and sits above 98% of US counties on that measure.