Sinkhole risk in Big Horn County, MT: 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: 11%
At or near the surface: 11%
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 555 of Big Horn County's 4,998 square miles on soluble rock, 555 of them with the rock at or near the surface. With about 12,796 residents (3 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 597 map points about 2.9 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Big Horn County

The nearest county rated sinkhole country is Fall River County, SD, about 249 miles southeast. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

Yellowstone County, MT 52 mi northwest · under 1% at surface
Sheridan County, WY 53 mi southeast · 15% at surface
Treasure County, MT 58 mi north · 0% at surface
Big Horn County, WY 65 mi south · 25% at surface
Rosebud County, MT 67 mi northeast · 0% at surface
Carbon County, MT 74 mi west · 12% at surface

What Big Horn County is built on

The largest mapped soluble unit here is the Mississippian, undifferentiated (carbonate), Mississippian in age, covering about 5% of the county. Next is the Triassic, undifferentiated at 3%.

3% 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 Montana

Big Horn County ranks 17 of 56 in Montana for soluble rock at the surface, and sits above 69% of US counties on that measure.