Sinkhole risk in Granite 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: 17%
At or near the surface: 17%
Buried under cover: 0%

What to do about a house in Granite 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 285 of Granite County's 1,727 square miles on soluble rock, 285 of them with the rock at or near the surface. With about 3,602 residents (2 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 278 map points about 2.5 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Granite County

The nearest county rated sinkhole country is Emery County, UT, about 528 miles south. None of the 6 closest counties land in the same band, so Granite County stands out from its surroundings.

Deer Lodge County, MT 25 mi southeast · 10% at surface
Powell County, MT 39 mi northeast · 13% at surface
Ravalli County, MT 39 mi southwest · 5% at surface
Missoula County, MT 49 mi northwest · 12% at surface
Silver Bow County, MT 50 mi southeast · 10% at surface
Jefferson County, MT 68 mi east · 6% at surface

What Granite County is built on

The largest mapped soluble unit here is the Newland limestone (carbonate), Middle-Proterozoic in age, covering about 8% of the county. Next is the Cambrian, undifferentiated at 5%.

1% 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

Granite County ranks 11 of 56 in Montana for soluble rock at the surface, and sits above 73% of US counties on that measure.