Sinkhole risk in Washington County, IN: Sinkhole country

Sinkhole country

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

On soluble rock: 63%
At or near the surface: 62%
Buried under cover: under 1%

The next step for one property

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

That works out to roughly 322 of Washington County's 514 square miles on soluble rock, 317 of them with the rock at or near the surface. With about 28,345 residents (55 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 337 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why gypsum and salt move faster than limestone, and the signs to look for and how an inspection works.

Counties around Washington County

The closest county with no soluble rock mapped is Brown County, IN, about 42 miles north. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Scott County, IN 20 mi east · 2% at surface
Orange County, IN 21 mi west · 90% at surface
Jackson County, IN 22 mi north · 2% at surface
Floyd County, IN 22 mi southeast · 29% at surface
Clark County, IN 23 mi east · 3% at surface
Lawrence County, IN 26 mi northwest · 71% 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 Sanders Group at 26%.

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

Washington County ranks 5 of 92 in Indiana for soluble rock at the surface, and sits above 90% of US counties on that measure.