SinkholeOdds

Sinkhole risk in Iosco County, MI: Sinkhole country

Sinkhole country

Fast-dissolving gypsum or salt beds under a large part of the county.

On soluble rock
40%
At or near the surface
0%
Buried under cover
40%

The next step for one property

Get a sinkhole or geotechnical inspection before buying, building or adding weight to a foundation.

That works out to roughly 219 of Iosco County's 549 square miles on soluble rock. With about 25,361 residents (46 per square mile), much of the land is open, and the mapped rock may sit far from where people actually live. The figures come from 346 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 40% under cover still counts.

Counties around Iosco County

The closest county with no soluble rock mapped is Bay County, MI, about 56 miles southwest. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

The rock underneath

The largest mapped soluble unit here is the Michigan Formation (black shale), Mississippian in age, covering about 40% of the county.

40% of the county sits on gypsum or salt, which dissolves in years rather than millennia. That share matters more than its size suggests.

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

Iosco County ranks 43 of 83 in Michigan for soluble rock at the surface, and sits above 0% of US counties on that measure.