Sinkhole risk in Orange County, IN: Sinkhole country

Sinkhole country

Soluble rock at or near the surface across much of the county.

On soluble rock: 92%
At or near the surface: 90%
Buried under cover: 2%

What to do about a house in Orange County

Get a sinkhole or geotechnical inspection before buying, building or adding weight to a foundation.

That works out to roughly 367 of Orange County's 398 square miles on soluble rock, 359 of them with the rock at or near the surface. With about 19,824 residents (50 per square mile), there is effectively nowhere in the county off the soluble rock, town or country. The figures come from 414 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 the signs to look for and how an inspection works.

Counties around Orange County

The closest county with no soluble rock mapped is Daviess County, IN, about 33 miles west. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Crawford County, IN 18 mi south · 83% at surface
Martin County, IN 20 mi northwest · 23% at surface
Lawrence County, IN 20 mi north · 71% at surface
Washington County, IN 21 mi east · 62% at surface
Dubois County, IN 24 mi southwest · 5% at surface
Harrison County, IN 33 mi southeast · 94% at surface

The rock underneath

The largest mapped soluble unit here is the Blue River Group (limestone), Mississippian in age, covering about 36% of the county. Next is the West Baden Group at 28%.

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 Indiana

Orange County ranks 2 of 92 in Indiana for soluble rock at the surface, and sits above 97% of US counties on that measure.