Sinkhole risk in Stillwater County, MT: No soluble rock mapped

No soluble rock mapped

No soluble bedrock mapped.

On soluble rock: under 1%
At or near the surface: under 1%
Buried under cover: 0%

What this means before you buy

No soluble bedrock is mapped here, so natural sinkholes are unlikely. Collapses from broken pipes or old mines are a separate risk.

That works out to roughly 7 of Stillwater County's 1,797 square miles on soluble rock, 7 of them with the rock at or near the surface. With about 9,450 residents (5 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 460 map points about 2.0 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Stillwater County

The nearest county rated sinkhole country is Fall River County, SD, about 335 miles southeast. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

Sweet Grass County, MT 29 mi west · 3% at surface
Carbon County, MT 35 mi southeast · 12% at surface
Golden Valley County, MT 51 mi north · 4% at surface
Yellowstone County, MT 57 mi east · under 1% at surface
Park County, MT 57 mi west · 6% at surface
Wheatland County, MT 62 mi north · 6% at surface

The rock underneath

The largest mapped soluble unit here is the Jurassic, undifferentiated (fine-grained mixed clastic), Jurassic in age, covering about under 1% of the county. Next is the Mississippian, undifferentiated at under 1%.

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

Stillwater County ranks 35 of 56 in Montana for soluble rock at the surface, and sits above 52% of US counties on that measure.