Sinkhole risk in Calhoun County, IL: Sinkhole country

Sinkhole country

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

On soluble rock: 75%
At or near the surface: 51%
Buried under cover: 25%

What to do about a house in Calhoun County

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

That works out to roughly 191 of Calhoun County's 254 square miles on soluble rock, 128 of them with the rock at or near the surface. With about 4,224 residents (17 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 309 map points about 0.9 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Calhoun County

The closest county with no soluble rock mapped is Macoupin County, IL, about 40 miles east. None of the 6 closest counties land in the same band, so Calhoun County stands out from its surroundings.

Jersey County, IL 17 mi east · 7% at surface
Lincoln County, MO 17 mi southwest · 0% at surface
Greene County, IL 20 mi northeast · 0% at surface
St. Charles County, MO 26 mi south · 14% at surface
Pike County, MO 30 mi northwest · 0% at surface
Pike County, IL 34 mi north · 12% at surface

What Calhoun County is built on

The largest mapped soluble unit here is the Lower Valmeyeran (Keokuk-Sedalia) Series (limestone), Mississippian in age, covering about 22% of the county. Next is the Kinderhookian Series at 17%.

Against the rest of Illinois

Calhoun County ranks 4 of 102 in Illinois for soluble rock at the surface, and sits above 88% of US counties on that measure.