SinkholeOdds

Sinkhole risk in Fergus 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 538 of Fergus County's 4,339 square miles on soluble rock, 538 of them with the rock at or near the surface. With about 11,862 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 251 map points about 4.2 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 Fergus County

The nearest county rated sinkhole country is Fall River County, SD, about 392 miles southeast. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

The rock underneath

The largest mapped soluble unit here is the Mississippian, undifferentiated (carbonate), Mississippian in age, covering about 7% of the county. Next is the Jurassic, undifferentiated at 6%.

6% of the county sits on gypsum or salt, which dissolves in years rather than millennia. That share matters more than its size suggests.

Against the rest of Montana

Fergus County ranks 14 of 56 in Montana for soluble rock at the surface, and sits above 70% of US counties on that measure.