Sinkhole risk in Gregory 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: 18%
At or near the surface: 17%
Buried under cover: under 1%

What this means before you buy

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 182 of Gregory County's 1,015 square miles on soluble rock, 174 of them with the rock at or near the surface. With about 4,110 residents (4 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 591 map points about 1.3 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 Gregory County

The nearest county rated sinkhole country is Boyd County, NE, about 29 miles southeast, close enough that the same rock may run under the county line. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

Boyd County, NE 29 mi southeast · 69% at surface
Charles Mix County, SD 31 mi east · 3% at surface
Keya Paha County, NE 33 mi southwest · 21% at surface
Tripp County, SD 36 mi west · 3% at surface
Brule County, SD 39 mi north · under 1% at surface
Douglas County, SD 45 mi east · 0% at surface

What Gregory County is built on

The largest mapped soluble unit here is the Pierre Shale (shale), Late Cretaceous in age, covering about 17% of the county. Next is the Niobrara Formation at under 1%.

17% 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

Gregory County ranks 4 of 66 in South Dakota for soluble rock at the surface, and sits above 73% of US counties on that measure.