SinkholeOdds

Sinkhole risk in Mason 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
31%
At or near the surface
0%
Buried under cover
31%

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 169 of Mason County's 539 square miles on soluble rock. With about 12,543 residents (23 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 560 map points about 1.0 miles apart, so a pocket smaller than that can be missed or overstated.

Read next: why gypsum and salt move faster than limestone, and why the 31% under cover still counts.

Counties around Mason County

The nearest county rated sinkhole country is Calhoun County, IL, about 84 miles southwest. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Which formation it is

The largest mapped soluble unit here is the Middle Valmeyeran (Salem, Warsaw, Borden, Springville; includes thin Mvl and Mk in south and east) Series (limestone), Mississippian in age, covering about 27% of the county. Next is the Lower Valmeyeran (Keokuk-Sedalia) Series at 2%.

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

Mason County ranks 76 of 102 in Illinois for soluble rock at the surface, and sits above 0% of US counties on that measure.