Sinkhole risk in Madison 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 107 of Madison County's 716 square miles on soluble rock. With about 263,017 residents (368 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 334 map points about 1.5 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Madison County

The nearest county rated sinkhole country is St. Louis County, MO, about 32 miles southwest, 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.

St. Louis city, MO 23 mi southwest · 47% at surface
St. Clair County, IL 25 mi south · 0% at surface
Bond County, IL 25 mi east · 0% at surface
Clinton County, IL 30 mi southeast · 0% at surface
Jersey County, IL 30 mi northwest · 7% at surface
Macoupin County, IL 30 mi north · 0% at surface

Which formation it is

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

Madison County ranks 73 of 102 in Illinois for soluble rock at the surface, and sits above 0% of US counties on that measure.