SinkholeOdds

Sinkhole risk in Arenac County, MI: Sinkhole country

Sinkhole country

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

On soluble rock
65%
At or near the surface
under 1%
Buried under cover
65%

What to do about a house in Arenac County

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

That works out to roughly 237 of Arenac County's 363 square miles on soluble rock, 1 of them with the rock at or near the surface. With about 15,087 residents (42 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 285 map points about 1.1 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 65% under cover still counts.

Counties around Arenac County

The closest county with no soluble rock mapped is Bay County, MI, about 26 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 65% of the county.

65% 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

Arenac County ranks 8 of 83 in Michigan for soluble rock at the surface, and sits above 52% of US counties on that measure.