Sinkhole risk in Massac 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: 21%
At or near the surface: 7%
Buried under cover: 14%

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 49 of Massac County's 237 square miles on soluble rock, 16 of them with the rock at or near the surface. With about 13,627 residents (57 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 556 map points about 0.7 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Massac County

The nearest county rated sinkhole country is Livingston County, KY, about 19 miles east, 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.

McCracken County, KY 11 mi south · 0% at surface
Pope County, IL 17 mi northeast · 19% at surface
Livingston County, KY 19 mi east · 54% at surface
Johnson County, IL 19 mi northwest · 19% at surface
Ballard County, KY 20 mi southwest · 0% at surface
Pulaski County, IL 23 mi west · 7% at surface

Which formation it is

The largest mapped soluble unit here is the Lower Chesterian (Glen Dean - Renault) Series (limestone), Mississippian in age, covering about 15% of the county. Next is the Upper Valmeyeran (Aux Vases, Ste. Genevieve, St. Louis) Series at 6%.

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

Massac County ranks 11 of 102 in Illinois for soluble rock at the surface, and sits above 65% of US counties on that measure.