SinkholeOdds

Sinkhole risk in Dallas 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
24%
At or near the surface
0%
Buried under cover
24%

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 144 of Dallas County's 588 square miles on soluble rock. With about 115,343 residents (196 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 373 map points about 1.3 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 24% under cover still counts.

Counties around Dallas County

The nearest county rated sinkhole country is Clayton County, IA, about 160 miles northeast. 4 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 21% of the county. Next is the Kansas City 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

Dallas County ranks 35 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.