SinkholeOdds

Sinkhole risk in Carter 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
4%
At or near the surface
4%
Buried under cover
0%

What to do about a house in Carter County

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

That works out to roughly 134 of Carter County's 3,340 square miles on soluble rock, 134 of them with the rock at or near the surface. With about 1,399 residents (0 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 354 map points about 3.1 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 Carter County

The nearest county rated sinkhole country is Fall River County, SD, about 166 miles south. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

What Carter County is built on

The largest mapped soluble unit here is the Niobrara formation (shale), Late Cretaceous in age, covering about 3% of the county. Next is the Mowry shale at under 1%.

The county also lies over the Williston 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

Carter County ranks 30 of 56 in Montana for soluble rock at the surface, and sits above 61% of US counties on that measure.