SinkholeOdds

Sinkhole risk in St. Clair County, MI: 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
17%
At or near the surface
under 1%
Buried under cover
17%

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 122 of St. Clair County's 722 square miles on soluble rock, 1 of them with the rock at or near the surface. With about 160,308 residents (222 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 602 map points about 1.1 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 17% under cover still counts.

Counties around St. Clair County

The nearest county rated sinkhole country is Lapeer County, MI, about 30 miles west, close enough that the same rock may run under the county line. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

The rock underneath

The largest mapped soluble unit here is the Bedford Shale (shale), Upper Devonian in age, covering about 17% of the county.

The county also lies over the Michigan Basin; Ohio Basin; Appalachian 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 Michigan

St. Clair County ranks 12 of 83 in Michigan for soluble rock at the surface, and sits above 50% of US counties on that measure.