Sinkhole risk in Hardin County, IA: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock: 95%
At or near the surface: 0%
Buried under cover: 95%

What this means before you buy

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 539 of Hardin County's 569 square miles on soluble rock. With about 16,464 residents (29 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 376 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why gypsum and salt move faster than limestone, and why the 95% under cover still counts.

Counties around Hardin County

The nearest county rated sinkhole country is Clayton County, IA, about 102 miles east. 5 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Grundy County, IA 23 mi east · 0% at surface
Hamilton County, IA 24 mi west · 0% at surface
Franklin County, IA 24 mi north · 0% at surface
Story County, IA 27 mi southwest · 0% at surface
Marshall County, IA 28 mi southeast · 0% at surface
Butler County, IA 33 mi northeast · 0% at surface

The rock underneath

The largest mapped soluble unit here is the Kinderhook Series (limestone), Early Mississippian in age, covering about 79% of the county. Next is the Osage Series at 9%.

The county also lies over the Forest City 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 Iowa

Hardin County ranks 48 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.