Sinkhole risk in Polk County, MO: Sinkhole country

Sinkhole country

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

On soluble rock: 97%
At or near the surface: 97%
Buried under cover: 0%

What to do about a house in Polk County

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

That works out to roughly 618 of Polk County's 636 square miles on soluble rock, 618 of them with the rock at or near the surface. With about 33,070 residents (52 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 368 map points about 1.3 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 Polk County

The closest county with no soluble rock mapped is Johnson County, AR, about 141 miles south. 5 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Dallas County, MO 21 mi east · 100% at surface
Hickory County, MO 23 mi north · 92% at surface
Greene County, MO 25 mi south · 97% at surface
Cedar County, MO 26 mi west · 54% at surface
Dade County, MO 28 mi southwest · 73% at surface
St. Clair County, MO 36 mi northwest · 43% at surface

What Polk 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 56% of the county. Next is the Kinderhookian Series at 24%.

Against the rest of Missouri

Polk County ranks 29 of 115 in Missouri for soluble rock at the surface, and sits above 98% of US counties on that measure.