SinkholeOdds

Sinkhole risk in Fremont County, WY: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock
9%
At or near the surface
9%
Buried under cover
0%

What this means before you buy

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 799 of Fremont County's 9,184 square miles on soluble rock, 799 of them with the rock at or near the surface. With about 39,721 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 612 map points about 3.9 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 how these percentages are measured.

Counties around Fremont County

The nearest county rated sinkhole country is Fall River County, SD, about 257 miles east. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Which formation it is

The largest mapped soluble unit here is the Bighorn Dolomite, Gallatin Limestone, and Gros Ventre Formation (TB), Bighorn Dolomite, Snowy Range Formation, Pilgrim Limestone, Park Shale, Meagher Limestone, Wolsey Shale, and Flathead Sandstone (Y), Bighorn Dolomite, Gallatin Limestone, Gros Ve (limestone), Middle Cambrian-Upper Ordovician in age, covering about 3% of the county. Next is the Tensleep Sandstone and Amsden Formation at 2%.

2% 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 Wind 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

Fremont County ranks 13 of 23 in Wyoming for soluble rock at the surface, and sits above 67% of US counties on that measure.