SinkholeOdds

Sinkhole risk in Johnson County, IL: Pockets of it

Pockets of it

Soluble rock at the surface or under thin cover in part of the county.

On soluble rock
26%
At or near the surface
19%
Buried under cover
7%

What to do about a house in Johnson County

Check the parcel against your state survey sinkhole map, and ask for an inspection if cracks, depressions or sticking doors show up.

That works out to roughly 88 of Johnson County's 344 square miles on soluble rock, 65 of them with the rock at or near the surface. With about 13,320 residents (39 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 360 map points about 1.0 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: why gypsum and salt move faster than limestone, and why the 7% under cover still counts.

Counties around Johnson County

The nearest county rated sinkhole country is Union County, IL, about 20 miles west, 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.

Which formation it is

The largest mapped soluble unit here is the Lower Chesterian (Glen Dean - Renault) Series (limestone), Mississippian in age, covering about 21% of the county. Next is the Upper Valmeyeran (Aux Vases, Ste. Genevieve, St. Louis) Series at 5%.

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

Johnson County ranks 7 of 102 in Illinois for soluble rock at the surface, and sits above 74% of US counties on that measure.