Sinkhole risk in Black Hawk 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: 100%
At or near the surface: 0%
Buried under cover: 100%

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 566 of Black Hawk County's 566 square miles on soluble rock. With about 132,348 residents (234 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 393 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why the 100% under cover still counts, and how these percentages are measured.

Counties around Black Hawk County

The nearest county rated sinkhole country is Clayton County, IA, about 56 miles northeast. 5 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Bremer County, IA 21 mi north · 0% at surface
Buchanan County, IA 24 mi east · 0% at surface
Grundy County, IA 25 mi west · 0% at surface
Benton County, IA 29 mi southeast · 0% at surface
Tama County, IA 30 mi southwest · 0% at surface
Butler County, IA 30 mi northwest · 0% at surface

Which formation it is

The largest mapped soluble unit here is the Cedar Valley Limestone (limestone), Middle Dovonian in age, covering about 73% of the county. Next is the Wapsipinicon Formation at 16%.

Against the rest of Iowa

Black Hawk County ranks 19 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.