SinkholeOdds

Sinkhole risk in Madison 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
68%
At or near the surface
0%
Buried under cover
68%

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 384 of Madison County's 561 square miles on soluble rock. With about 17,161 residents (31 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 393 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 68% under cover still counts.

Counties around Madison County

The nearest county rated sinkhole country is Jackson County, MO, about 162 miles south. 6 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

What Madison County is built on

The largest mapped soluble unit here is the Kansas City Group (limestone), Late Pennsylvanian-Missouri Series in age, covering about 66% of the county. Next is the Lansing Group at 2%.

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

Madison County ranks 63 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.