Sinkhole risk in Hardin County, IL: Sinkhole country

Sinkhole country

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

On soluble rock: 66%
At or near the surface: 61%
Buried under cover: 6%

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 117 of Hardin County's 177 square miles on soluble rock, 107 of them with the rock at or near the surface. With about 3,550 residents (20 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 284 map points about 0.8 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: why gypsum and salt move faster than limestone, and why the 6% under cover still counts.

Counties around Hardin County

The closest county with no soluble rock mapped is Gallatin County, IL, about 17 miles north. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Crittenden County, KY 14 mi southeast · 66% at surface
Pope County, IL 17 mi southwest · 19% at surface
Gallatin County, IL 17 mi north · 0% at surface
Union County, KY 20 mi northeast · 0% at surface
Livingston County, KY 22 mi south · 54% at surface
Saline County, IL 22 mi northwest · under 1% at surface

Which formation it is

The largest mapped soluble unit here is the Upper Valmeyeran (Aux Vases, Ste. Genevieve, St. Louis) Series (limestone), Mississippian in age, covering about 30% of the county. Next is the Lower Chesterian (Glen Dean - Renault) Series at 27%.

The county also lies over the Illinois Basin, a deep salt or gypsum basin. Deep beds rarely cause surface collapse on their own, but wells and mining into them have.

Against the rest of Illinois

Hardin County ranks 1 of 102 in Illinois for soluble rock at the surface, and sits above 90% of US counties on that measure.