SinkholeOdds

Sinkhole risk in Monroe County, MI: Sinkhole country

Sinkhole country

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

On soluble rock
94%
At or near the surface
under 1%
Buried under cover
94%

What to do about a house in Monroe County

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

That works out to roughly 516 of Monroe County's 549 square miles on soluble rock, 2 of them with the rock at or near the surface. With about 156,045 residents (284 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 309 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 94% under cover still counts.

Counties around Monroe County

The closest county with no soluble rock mapped is Fulton County, OH, about 40 miles southwest. None of the 6 closest counties land in the same band, so Monroe County stands out from its surroundings.

Which formation it is

The largest mapped soluble unit here is the Detroit River Group (dolostone (dolomite)), Middle Devonian in age, covering about 29% of the county. Next is the Bass Island Dolomite at 25%.

36% 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

Monroe County ranks 9 of 83 in Michigan for soluble rock at the surface, and sits above 51% of US counties on that measure.