Sinkhole risk in Lewis and Clark 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 Lewis and Clark 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 878 of Lewis and Clark County's 3,458 square miles on soluble rock, 878 of them with the rock at or near the surface. With about 75,129 residents (22 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 433 map points about 2.8 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Lewis and Clark County

The nearest county rated sinkhole country is Fall River County, SD, about 509 miles southeast. 3 of the 6 closest counties land in the same band; the ground changes quickly around here.

Powell County, MT 32 mi southwest · 13% at surface
Teton County, MT 48 mi north · 9% at surface
Cascade County, MT 50 mi east · 18% at surface
Broadwater County, MT 69 mi southeast · 27% at surface
Granite County, MT 70 mi southwest · 17% at surface
Jefferson County, MT 71 mi south · 6% at surface

Which formation it is

The largest mapped soluble unit here is the Mississippian, undifferentiated (carbonate), Mississippian in age, covering about 8% of the county. Next is the Helena limestone at 6%.

Against the rest of Montana

Lewis and Clark County ranks 6 of 56 in Montana for soluble rock at the surface, and sits above 77% of US counties on that measure.