SinkholeOdds

Sinkhole risk in Grundy 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
99%
At or near the surface
0%
Buried under cover
99%

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 497 of Grundy County's 502 square miles on soluble rock. With about 12,405 residents (25 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 346 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why gypsum and salt move faster than limestone, and why the 99% under cover still counts.

Counties around Grundy County

The nearest county rated sinkhole country is Clayton County, IA, about 80 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 Lime Creek Formation (limestone), Late Devonian in age, covering about 40% of the county. Next is the Kinderhook Series at 26%.

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

Grundy County ranks 44 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.