SinkholeOdds

Sinkhole risk in Kent County, MI: Sinkhole country

Sinkhole country

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

On soluble rock
49%
At or near the surface
0%
Buried under cover
49%

What to do about a house in Kent County

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

That works out to roughly 420 of Kent County's 850 square miles on soluble rock. With about 673,002 residents (792 per square mile), most of the county is built up, so a mapped unit is likely to sit under somebody's street. The figures come from 399 map points about 1.5 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 49% under cover still counts.

Counties around Kent County

The closest county with no soluble rock mapped is Montcalm County, MI, about 28 miles northeast. 2 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 30% of the county. Next is the Bayport Limestone at 19%.

30% 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

Kent County ranks 49 of 83 in Michigan for soluble rock at the surface, and sits above 0% of US counties on that measure.