Sinkhole risk in Pondera 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: 6%
At or near the surface: 6%
Buried under cover: 0%

The next step for one property

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 97 of Pondera County's 1,625 square miles on soluble rock, 97 of them with the rock at or near the surface. With about 6,078 residents (4 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 554 map points about 1.7 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: how these percentages are measured, and the warning signs that apply anywhere.

Counties around Pondera County

The nearest county rated sinkhole country is Fall River County, SD, about 544 miles southeast. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Teton County, MT 28 mi south · 9% at surface
Toole County, MT 36 mi northeast · 0% at surface
Glacier County, MT 48 mi northwest · 5% at surface
Liberty County, MT 59 mi northeast · under 1% at surface
Cascade County, MT 75 mi southeast · 18% at surface
Lewis and Clark County, MT 77 mi south · 25% at surface

What Pondera County is built on

The largest mapped soluble unit here is the Mississippian, undifferentiated (carbonate), Mississippian in age, covering about 4% of the county. Next is the Devonian, undifferentiated at 1%.

Against the rest of Montana

Pondera County ranks 23 of 56 in Montana for soluble rock at the surface, and sits above 64% of US counties on that measure.