Sinkhole risk in Carroll 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: 77%
At or near the surface: 11%
Buried under cover: 66%

What to do about a house in Carroll 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 342 of Carroll County's 445 square miles on soluble rock, 47 of them with the rock at or near the surface. With about 15,444 residents (35 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 310 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why the 66% under cover still counts, and the signs to look for and how an inspection works.

Counties around Carroll County

The nearest county rated sinkhole country is Jo Daviess County, IL, about 25 miles northwest, close enough that the same rock may run under the county line. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Whiteside County, IL 22 mi south · under 1% at surface
Stephenson County, IL 23 mi northeast · 3% at surface
Jo Daviess County, IL 25 mi northwest · 58% at surface
Ogle County, IL 31 mi east · 7% at surface
Clinton County, IA 34 mi west · 1% at surface
Jackson County, IA 34 mi west · 33% at surface

Which formation it is

The largest mapped soluble unit here is the Galena-Platteville Groups (limestone), Ordovician in age, covering about 44% of the county. Next is the Silurian at 33%.

Against the rest of Illinois

Carroll County ranks 9 of 102 in Illinois for soluble rock at the surface, and sits above 68% of US counties on that measure.