SinkholeOdds

Sinkhole risk in Washtenaw County, MI: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock
12%
At or near the surface
0%
Buried under cover
12%

What this means before you buy

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 87 of Washtenaw County's 706 square miles on soluble rock. With about 373,875 residents (530 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 373 map points about 1.4 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 12% under cover still counts.

Counties around Washtenaw County

The nearest county rated sinkhole country is Monroe County, MI, about 30 miles southeast, close enough that the same rock may run under the county line. 5 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

The rock underneath

The largest mapped soluble unit here is the Traverse Group (limestone), Middle Devonian in age, covering about 6% of the county. Next is the Bedford Shale at 4%.

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

Washtenaw County ranks 81 of 83 in Michigan for soluble rock at the surface, and sits above 0% of US counties on that measure.