SinkholeOdds

Sinkhole risk in Marshall 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
73%
At or near the surface
0%
Buried under cover
73%

What to do about a house in Marshall County

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

That works out to roughly 418 of Marshall County's 573 square miles on soluble rock. With about 40,392 residents (71 per square mile), homes are spread across towns and farmland, and a mapped unit may or may not reach the developed parts. The figures come from 391 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 why the 73% under cover still counts.

Counties around Marshall 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.

Which formation it is

The largest mapped soluble unit here is the Osage Series (dolostone (dolomite)), Early Mississippian in age, covering about 40% of the county. Next is the Kinderhook Series at 16%.

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

Marshall County ranks 66 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.