SinkholeOdds

Sinkhole risk in Charles Mix County, SD: 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
35%
At or near the surface
3%
Buried under cover
32%

What to do about a house in Charles Mix County

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

That works out to roughly 381 of Charles Mix County's 1,097 square miles on soluble rock, 35 of them with the rock at or near the surface. With about 9,343 residents (9 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 406 map points about 1.6 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why gypsum and salt move faster than limestone, and why the 32% under cover still counts.

Counties around Charles Mix County

The nearest county rated sinkhole country is Boyd County, NE, about 23 miles southwest, close enough that the same rock may run under the county line. 3 of the 6 closest counties land in the same band; the ground changes quickly around here.

Which formation it is

The largest mapped soluble unit here is the Niobrara Formation (limestone), Late Cretaceous in age, covering about 31% of the county. Next is the Pierre Shale at 5%.

5% 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 South Dakota

Charles Mix County ranks 9 of 66 in South Dakota for soluble rock at the surface, and sits above 60% of US counties on that measure.