Sinkhole risk in Boyd County, NE: Sinkhole country

Sinkhole country

Fast-dissolving gypsum or salt beds under a large part of the county.

On soluble rock: 72%
At or near the surface: 69%
Buried under cover: 2%

What to do about a house in Boyd County

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

That works out to roughly 386 of Boyd County's 539 square miles on soluble rock, 374 of them with the rock at or near the surface. With about 1,700 residents (3 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 277 map points about 1.4 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 Boyd County

The closest county with no soluble rock mapped is Antelope County, NE, about 61 miles southeast. None of the 6 closest counties land in the same band, so Boyd County stands out from its surroundings.

Charles Mix County, SD 23 mi northeast · 3% at surface
Gregory County, SD 29 mi northwest · 17% at surface
Holt County, NE 30 mi south · 15% at surface
Douglas County, SD 40 mi northeast · 0% at surface
Bon Homme County, SD 45 mi east · under 1% at surface
Rock County, NE 48 mi southwest · 5% at surface

Which formation it is

The largest mapped soluble unit here is the Pierre Shale (shale), Late Cretaceous in age, covering about 69% of the county. Next is the Niobrara Formation at 3%.

69% 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 Nebraska

Boyd County ranks 6 of 93 in Nebraska for soluble rock at the surface, and sits above 92% of US counties on that measure.