SinkholeOdds

Sinkhole risk in Roscommon County, MI: Sinkhole country

Sinkhole country

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

On soluble rock
54%
At or near the surface
0%
Buried under cover
54%

What this means before you buy

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

That works out to roughly 281 of Roscommon County's 520 square miles on soluble rock. With about 23,932 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 391 map points about 1.2 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 54% under cover still counts.

Counties around Roscommon County

The closest county with no soluble rock mapped is Clare County, MI, about 27 miles southwest. 3 of the 6 closest counties land in the same band; the ground changes quickly around here.

Which formation it is

The largest mapped soluble unit here is the Michigan Formation (black shale), Mississippian in age, covering about 48% of the county. Next is the Bayport Limestone at 6%.

48% 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

Roscommon County ranks 74 of 83 in Michigan for soluble rock at the surface, and sits above 0% of US counties on that measure.