Sinkhole risk in Putnam County, IN: 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: 50%
At or near the surface: under 1%
Buried under cover: 50%

What this means before you buy

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

That works out to roughly 240 of Putnam County's 481 square miles on soluble rock, 2 of them with the rock at or near the surface. With about 37,804 residents (79 per square mile), homes are spread across towns and farmland, and a mapped unit may or may not reach the developed parts. The figures come from 368 map points about 1.1 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 why the 50% under cover still counts.

Counties around Putnam County

The nearest county rated sinkhole country is Monroe County, IN, about 39 miles southeast, 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.

Hendricks County, IN 20 mi east · 0% at surface
Parke County, IN 20 mi west · under 1% at surface
Clay County, IN 23 mi southwest · 0% at surface
Owen County, IN 24 mi south · 16% at surface
Morgan County, IN 25 mi southeast · 2% at surface
Montgomery County, IN 26 mi north · 0% at surface

The rock underneath

The largest mapped soluble unit here is the Blue River Group (limestone), Mississippian in age, covering about 32% of the county. Next is the Sanders Group at 19%.

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

Putnam County ranks 27 of 92 in Indiana for soluble rock at the surface, and sits above 53% of US counties on that measure.