Sinkhole risk in Union County, IL: Sinkhole country

Sinkhole country

Soluble rock at or near the surface across much of the county.

On soluble rock: 74%
At or near the surface: 53%
Buried under cover: 20%

What to do about a house in Union County

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

That works out to roughly 304 of Union County's 413 square miles on soluble rock, 221 of them with the rock at or near the surface. With about 16,886 residents (41 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 352 map points about 1.1 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Union County

The closest county with no soluble rock mapped is Williamson County, IL, about 24 miles northeast. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

Pulaski County, IL 20 mi south · 7% at surface
Johnson County, IL 20 mi east · 19% at surface
Alexander County, IL 22 mi south · 31% at surface
Jackson County, IL 22 mi north · under 1% at surface
Williamson County, IL 24 mi northeast · 0% at surface
Cape Girardeau County, MO 25 mi west · 81% at surface

Which formation it is

The largest mapped soluble unit here is the Lower Devonian (limestone), Devonian in age, covering about 26% of the county. Next is the Upper Valmeyeran (Aux Vases, Ste. Genevieve, St. Louis) Series at 16%.

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

Union County ranks 3 of 102 in Illinois for soluble rock at the surface, and sits above 88% of US counties on that measure.