SinkholeOdds

Sinkhole risk in Big Horn County, WY: Pockets of it

Pockets of it

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

On soluble rock
25%
At or near the surface
25%
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 794 of Big Horn County's 3,137 square miles on soluble rock, 794 of them with the rock at or near the surface. With about 12,084 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 293 map points about 3.3 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 Big Horn County

The nearest county rated sinkhole country is Fall River County, SD, about 241 miles east. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

The rock underneath

The largest mapped soluble unit here is the Madison Group and Darby Formation (TB, N) or Madison Group and Three Forks and Jefferson Formations (Y) (limestone), Upper Devonian-Upper Mississippian in age, covering about 7% of the county. Next is the Tensleep Sandstone and Amsden Formation at 6%.

8% of the county sits on gypsum or salt, which dissolves in years rather than millennia. That share matters more than its size suggests.

The county also lies over the Bighorn Basin, a deep salt or gypsum basin. Deep beds rarely cause surface collapse on their own, but wells and mining into them have.

Against the rest of Wyoming

Big Horn County ranks 4 of 23 in Wyoming for soluble rock at the surface, and sits above 77% of US counties on that measure.