SinkholeOdds

Sinkhole risk in Johnson 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
15%
At or near the surface
15%
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 623 of Johnson County's 4,154 square miles on soluble rock, 623 of them with the rock at or near the surface. With about 8,803 residents (2 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 341 map points about 3.5 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 Johnson County

The nearest county rated sinkhole country is Fall River County, SD, about 164 miles east. 3 of the 6 closest counties land in the same band; the ground changes quickly around here.

The rock underneath

The largest mapped soluble unit here is the Tensleep Sandstone and Amsden Formation (sandstone), Upper Mississippian-Permian in age, covering about 4% of the county. Next is the Chugwater and Goose Egg Formations at 4%.

4% 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 Powder River 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

Johnson County ranks 9 of 23 in Wyoming for soluble rock at the surface, and sits above 71% of US counties on that measure.