Sinkhole risk in Franklin County, NE: Sinkhole country

Sinkhole country

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

On soluble rock: 78%
At or near the surface: 78%
Buried under cover: 0%

The next step for one property

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

That works out to roughly 446 of Franklin County's 576 square miles on soluble rock, 446 of them with the rock at or near the surface. With about 2,817 residents (5 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 414 map points about 1.2 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Franklin County

The closest county with no soluble rock mapped is Phelps County, NE, about 33 miles northwest. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Kearney County, NE 23 mi north · 47% at surface
Harlan County, NE 23 mi west · 37% at surface
Webster County, NE 24 mi east · 78% at surface
Smith County, KS 29 mi south · 40% at surface
Phelps County, NE 33 mi northwest · 0% at surface
Adams County, NE 34 mi northeast · 79% at surface

What Franklin County is built on

The largest mapped soluble unit here is the Niobrara Formation (limestone), Late Cretaceous in age, covering about 49% of the county. Next is the Pierre Shale at 29%.

29% 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

Franklin County ranks 5 of 93 in Nebraska for soluble rock at the surface, and sits above 94% of US counties on that measure.