SinkholeOdds

Sinkhole risk in Cass 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%

The next step for one property

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 116 of Cass County's 376 square miles on soluble rock. With about 12,713 residents (34 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 279 map points about 1.2 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 31% under cover still counts.

Counties around Cass County

The nearest county rated sinkhole country is Calhoun County, IL, about 60 miles south. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

What Cass County is built on

The largest mapped soluble unit here is the Lower Valmeyeran (Keokuk-Sedalia) Series (limestone), Mississippian in age, covering about 23% of the county. Next is the Middle Valmeyeran (Salem, Warsaw, Borden, Springville; includes thin Mvl and Mk in south and east) Series at 7%.

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

Cass County ranks 32 of 102 in Illinois for soluble rock at the surface, and sits above 0% of US counties on that measure.