Sinkhole risk in Knox County, NE: Sinkhole country

Sinkhole country

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

On soluble rock: 56%
At or near the surface: 22%
Buried under cover: 33%

What this means before you buy

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

That works out to roughly 616 of Knox County's 1,108 square miles on soluble rock, 247 of them with the rock at or near the surface. With about 8,306 residents (7 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 354 map points about 1.8 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 why the 33% under cover still counts.

Counties around Knox County

The closest county with no soluble rock mapped is Antelope County, NE, about 32 miles south. None of the 6 closest counties land in the same band, so Knox County stands out from its surroundings.

Bon Homme County, SD 24 mi north · under 1% at surface
Pierce County, NE 29 mi southeast · 0% at surface
Antelope County, NE 32 mi south · 0% at surface
Cedar County, NE 32 mi east · 0% at surface
Yankton County, SD 36 mi northeast · 0% at surface
Holt County, NE 47 mi west · 15% at surface

The rock underneath

The largest mapped soluble unit here is the Pierre Shale (shale), Late Cretaceous in age, covering about 50% of the county. Next is the Niobrara Formation at 6%.

50% 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

Knox County ranks 15 of 93 in Nebraska for soluble rock at the surface, and sits above 76% of US counties on that measure.