Sinkhole risk in Flathead 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: 25%
At or near the surface: 25%
Buried under cover: 0%

What to do about a house in Flathead 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 1,282 of Flathead County's 5,087 square miles on soluble rock, 1,282 of them with the rock at or near the surface. With about 114,527 residents (23 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 567 map points about 3.0 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: the signs to look for and how an inspection works, and how these percentages are measured.

Counties around Flathead County

The nearest county rated sinkhole country is Fall River County, SD, about 616 miles southeast. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Lake County, MT 45 mi south · 15% at surface
Glacier County, MT 56 mi northeast · 5% at surface
Sanders County, MT 64 mi southwest · 20% at surface
Lincoln County, MT 67 mi west · 29% at surface
Pondera County, MT 84 mi east · 6% at surface
Missoula County, MT 88 mi south · 12% at surface

Which formation it is

The largest mapped soluble unit here is the Piegan group (carbonate), Middle-Proterozoic in age, covering about 10% of the county. Next is the Wallace formation at 8%.

Against the rest of Montana

Flathead County ranks 7 of 56 in Montana for soluble rock at the surface, and sits above 77% of US counties on that measure.