Sinkhole risk in Polk County, MN: 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: 10%
At or near the surface: 0%
Buried under cover: 10%

What this means before you buy

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

That works out to roughly 189 of Polk County's 1,971 square miles on soluble rock. With about 30,413 residents (15 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 593 map points about 1.8 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: why the 10% under cover still counts, and how these percentages are measured.

Counties around Polk County

The nearest county rated sinkhole country is Boyd County, NE, about 356 miles south. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

Red Lake County, MN 16 mi northeast · 0% at surface
Pennington County, MN 26 mi northeast · 0% at surface
Norman County, MN 31 mi south · 0% at surface
Marshall County, MN 41 mi north · 0% at surface
Mahnomen County, MN 41 mi southeast · 0% at surface
Traill County, ND 42 mi southwest · 0% at surface

The rock underneath

The largest mapped soluble unit here is the Middle and Upper Ordovician rocks, undivided (sandstone), Middle and Late Ordovician in age, covering about 9% of the county. Next is the Ordovician rocks at under 1%.

Against the rest of Minnesota

Polk County ranks 62 of 87 in Minnesota for soluble rock at the surface, and sits above 0% of US counties on that measure.