Sinkhole risk in Jefferson County, MO: Sinkhole country

Sinkhole country

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

On soluble rock: 99%
At or near the surface: 98%
Buried under cover: under 1%

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 646 of Jefferson County's 656 square miles on soluble rock, 644 of them with the rock at or near the surface. With about 231,888 residents (353 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 270 map points about 1.6 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 Jefferson County

The closest county with no soluble rock mapped is Washington County, IL, about 61 miles east. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Monroe County, IL 20 mi east · 6% at surface
St. Louis County, MO 27 mi north · 53% at surface
Washington County, MO 29 mi southwest · 96% at surface
Franklin County, MO 31 mi west · 96% at surface
St. Louis city, MO 31 mi northeast · 47% at surface
St. Francois County, MO 31 mi south · 71% at surface

What Jefferson County is built on

The largest mapped soluble unit here is the Smithville Dolomite, Powell Dolomite, Cotter Dolomite, Jefferson City Dolomite (dolostone (dolomite)), Early Ordovician-Ibexian Series in age, covering about 45% of the county. Next is the Osagean Series at 9%.

Against the rest of Missouri

Jefferson County ranks 25 of 115 in Missouri for soluble rock at the surface, and sits above 99% of US counties on that measure.