Sinkhole risk in Cascade County, MT: Pockets of it

Pockets of it

Soluble rock at the surface or under thin cover in part of the county.

On soluble rock: 18%
At or near the surface: 18%
Buried under cover: 0%

What to do about a house in Cascade County

Check the parcel against your state survey sinkhole map, and ask for an inspection if cracks, depressions or sticking doors show up.

That works out to roughly 494 of Cascade County's 2,698 square miles on soluble rock, 494 of them with the rock at or near the surface. With about 84,523 residents (31 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 268 map points about 3.2 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 the signs to look for and how an inspection works.

Counties around Cascade County

The nearest county rated sinkhole country is Fall River County, SD, about 474 miles southeast. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Lewis and Clark County, MT 50 mi west · 25% at surface
Judith Basin County, MT 53 mi east · 33% at surface
Meagher County, MT 54 mi south · 49% at surface
Teton County, MT 56 mi northwest · 9% at surface
Chouteau County, MT 58 mi northeast · 0% at surface
Broadwater County, MT 69 mi south · 27% at surface

The rock underneath

The largest mapped soluble unit here is the Mississippian, undifferentiated (carbonate), Mississippian in age, covering about 12% of the county. Next is the Jurassic, undifferentiated at 3%.

3% 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

Cascade County ranks 9 of 56 in Montana for soluble rock at the surface, and sits above 73% of US counties on that measure.