Sinkhole risk in Custer County, SD: Pockets of it

Pockets of it

Soluble rock at the surface or under thin cover in part of the county.

On soluble rock: 30%
At or near the surface: 30%
Buried under cover: 0%

The next step for one property

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 473 of Custer County's 1,557 square miles on soluble rock, 473 of them with the rock at or near the surface. With about 9,330 residents (6 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 339 map points about 2.1 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Custer County

The nearest county rated sinkhole country is Fall River County, SD, about 32 miles south, close enough that the same rock may run under the county line. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Fall River County, SD 32 mi south · 37% at surface
Pennington County, SD 39 mi northeast · 13% at surface
Lawrence County, SD 49 mi north · 42% at surface
Oglala Lakota County, SD 51 mi southeast · 4% at surface
Weston County, WY 56 mi west · 14% at surface
Niobrara County, WY 66 mi southwest · 15% at surface

What Custer County is built on

The largest mapped soluble unit here is the Minnelusa Formation (sandstone), Permian to Pennsylvanian in age, covering about 17% of the county. Next is the Madison Group at 7%.

6% 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

Custer County ranks 3 of 66 in South Dakota for soluble rock at the surface, and sits above 79% of US counties on that measure.