Sinkhole risk in Tama 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%

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 721 of Tama County's 721 square miles on soluble rock. With about 16,738 residents (23 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 399 map points about 1.3 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 100% under cover still counts.

Counties around Tama County

The nearest county rated sinkhole country is Clayton County, IA, about 81 miles northeast. 6 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Marshall County, IA 23 mi west · 0% at surface
Benton County, IA 24 mi east · 0% at surface
Grundy County, IA 26 mi northwest · 0% at surface
Poweshiek County, IA 27 mi south · 0% at surface
Black Hawk County, IA 30 mi north · 0% at surface
Iowa County, IA 36 mi southeast · 0% at surface

The rock underneath

The largest mapped soluble unit here is the Lime Creek Formation (limestone), Late Devonian in age, covering about 58% of the county. Next is the Cedar Valley Limestone at 21%.

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

Tama County ranks 87 of 99 in Iowa for soluble rock at the surface, and sits above 0% of US counties on that measure.