Sinkhole risk in St. Clair County, IL: 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: 15%
At or near the surface: 0%
Buried under cover: 15%

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 100 of St. Clair County's 658 square miles on soluble rock. With about 251,149 residents (382 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 290 map points about 1.5 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 15% under cover still counts.

Counties around St. Clair County

The nearest county rated sinkhole country is St. Louis County, MO, about 30 miles northwest, close enough that the same rock may run under the county line. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Monroe County, IL 19 mi southwest · 6% at surface
St. Louis city, MO 21 mi northwest · 47% at surface
Madison County, IL 25 mi north · 0% at surface
Clinton County, IL 29 mi east · 0% at surface
Washington County, IL 29 mi east · 0% at surface
Randolph County, IL 29 mi south · under 1% at surface

What St. Clair 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 10% of the county. Next is the Lower Chesterian (Glen Dean - Renault) Series at 4%.

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

St. Clair County ranks 88 of 102 in Illinois for soluble rock at the surface, and sits above 0% of US counties on that measure.