Sinkhole risk in Franklin 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: 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 887 of Franklin County's 922 square miles on soluble rock, 884 of them with the rock at or near the surface. With about 107,256 residents (116 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 309 map points about 1.7 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 Franklin County

The closest county with no soluble rock mapped is Macoupin County, IL, about 85 miles northeast. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Gasconade County, MO 24 mi west · 97% at surface
Warren County, MO 25 mi north · 42% at surface
Jefferson County, MO 31 mi east · 98% at surface
St. Charles County, MO 33 mi northeast · 14% at surface
Crawford County, MO 34 mi southwest · 100% at surface
Washington County, MO 34 mi south · 96% at surface

What Franklin 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 Roubidoux Formation at 23%.

Against the rest of Missouri

Franklin County ranks 32 of 115 in Missouri for soluble rock at the surface, and sits above 98% of US counties on that measure.