Sinkhole risk in Pennington County, SD: 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: 13%
At or near the surface: 13%
Buried under cover: 0%

What to do about a house in Pennington County

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

That works out to roughly 355 of Pennington County's 2,777 square miles on soluble rock, 355 of them with the rock at or near the surface. With about 115,979 residents (42 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 485 map points about 2.4 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 how these percentages are measured.

Counties around Pennington County

The nearest county rated sinkhole country is Fall River County, SD, about 64 miles southwest. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Custer County, SD 39 mi southwest · 30% at surface
Meade County, SD 42 mi north · 3% at surface
Oglala Lakota County, SD 48 mi south · 4% at surface
Lawrence County, SD 54 mi northwest · 42% at surface
Jackson County, SD 64 mi east · 0% at surface
Fall River County, SD 64 mi southwest · 37% at surface

What Pennington County is built on

The largest mapped soluble unit here is the Madison Group (limestone), Mississippian to Devonian in age, covering about 6% of the county. Next is the Minnelusa Formation at 5%.

1% of the county sits on gypsum or salt, which dissolves in years rather than millennia. That share matters more than its size suggests.

Against the rest of South Dakota

Pennington County ranks 5 of 66 in South Dakota for soluble rock at the surface, and sits above 70% of US counties on that measure.