Sinkhole risk in Monroe County, IL: Pockets of it

Pockets of it

Soluble rock at the surface or under thin cover in part of the county.

On soluble rock: 87%
At or near the surface: 6%
Buried under cover: 81%

What to do about a house in Monroe County

Check the parcel against your state survey sinkhole map, and ask for an inspection if cracks, depressions or sticking doors show up.

That works out to roughly 334 of Monroe County's 385 square miles on soluble rock, 22 of them with the rock at or near the surface. With about 34,969 residents (91 per square mile), homes are spread across towns and farmland, and a mapped unit may or may not reach the developed parts. The figures come from 535 map points about 0.8 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Monroe County

The nearest county rated sinkhole country is Jefferson County, MO, about 20 miles west, close enough that the same rock may run under the county line. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

St. Clair County, IL 19 mi northeast · 0% at surface
Jefferson County, MO 20 mi west · 98% at surface
Randolph County, IL 25 mi southeast · under 1% at surface
St. Louis city, MO 25 mi north · 47% at surface
Ste. Genevieve County, MO 27 mi south · 76% at surface
St. Louis County, MO 29 mi northwest · 53% at surface

What Monroe County is built on

The largest mapped soluble unit here is the Upper Valmeyeran (Aux Vases, Ste. Genevieve, St. Louis) Series (limestone), Mississippian in age, covering about 36% of the county. Next is the Lower Chesterian (Glen Dean - Renault) Series at 19%.

The county also lies over the Illinois 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 Illinois

Monroe County ranks 15 of 102 in Illinois for soluble rock at the surface, and sits above 64% of US counties on that measure.