SinkholeOdds

Sinkhole risk in Polk 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
5%
At or near the surface
0%
Buried under cover
5%

What to do about a house in Polk County

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

That works out to roughly 26 of Polk County's 573 square miles on soluble rock. With about 516,185 residents (901 per square mile), most of the county is built up, so a mapped unit is likely to sit under somebody's street. The figures come from 374 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Read next: why gypsum and salt move faster than limestone, and how these percentages are measured.

Counties around Polk County

The nearest county rated sinkhole country is Clayton County, IA, about 140 miles northeast. 5 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Which formation it is

The largest mapped soluble unit here is the Marmaton Group (shale), Middle Pennsylvanian-Des Moines Series in age, covering about 5% of the county.

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

Polk County ranks 79 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.