Sinkhole risk in Carbon 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: 12%
At or near the surface: 12%
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 246 of Carbon County's 2,048 square miles on soluble rock, 246 of them with the rock at or near the surface. With about 11,498 residents (6 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 585 map points about 1.9 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Carbon County

The nearest county rated sinkhole country is Fall River County, SD, about 306 miles southeast. 3 of the 6 closest counties land in the same band; the ground changes quickly around here.

Stillwater County, MT 35 mi northwest · under 1% at surface
Park County, WY 58 mi southwest · 1% at surface
Sweet Grass County, MT 60 mi northwest · 3% at surface
Yellowstone County, MT 61 mi northeast · under 1% at surface
Big Horn County, WY 70 mi southeast · 25% at surface
Big Horn County, MT 74 mi east · 11% at surface

Which formation it is

The largest mapped soluble unit here is the Mississippian, undifferentiated (carbonate), Mississippian in age, covering about 6% of the county. Next is the Triassic, undifferentiated at 3%.

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 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 Montana

Carbon County ranks 15 of 56 in Montana for soluble rock at the surface, and sits above 69% of US counties on that measure.