Sinkhole risk in Sweet Grass 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: 3%
At or near the surface: 3%
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 54 of Sweet Grass County's 1,856 square miles on soluble rock, 54 of them with the rock at or near the surface. With about 3,757 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 241 map points about 2.8 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Sweet Grass County

The nearest county rated sinkhole country is Fall River County, SD, about 364 miles southeast. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Stillwater County, MT 29 mi east · under 1% at surface
Park County, MT 37 mi southwest · 6% at surface
Wheatland County, MT 48 mi north · 6% at surface
Golden Valley County, MT 54 mi northeast · 4% at surface
Carbon County, MT 60 mi southeast · 12% at surface
Gallatin County, MT 62 mi west · 18% at surface

Which formation it is

The largest mapped soluble unit here is the Mississippian, undifferentiated (carbonate), Mississippian in age, covering about 1% of the county. Next is the Cambrian, undifferentiated at under 1%.

The county also lies over the Crazy Mountains 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

Sweet Grass County ranks 31 of 56 in Montana for soluble rock at the surface, and sits above 60% of US counties on that measure.