SinkholeOdds

Sinkhole risk in Poweshiek 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
89%
At or near the surface
0%
Buried under cover
89%

The next step for one property

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

That works out to roughly 519 of Poweshiek County's 585 square miles on soluble rock. With about 18,454 residents (32 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 391 map points about 1.2 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 89% under cover still counts.

Counties around Poweshiek County

The nearest county rated sinkhole country is Clayton County, IA, about 101 miles northeast. 6 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

The rock underneath

The largest mapped soluble unit here is the Osage Series (dolostone (dolomite)), Early Mississippian in age, covering about 37% of the county. Next is the Lime Creek Formation at 18%.

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

Poweshiek County ranks 81 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.