SinkholeOdds

Sinkhole risk in Slope County, ND: Not karst, but voids possible

Not karst, but voids possible

No soluble bedrock; some pseudokarst (piping or lava-tube ground).

On soluble rock
under 1%
At or near the surface
under 1%
Buried under cover
0%

What to do about a house in Slope County

Not true karst, but soil piping or lava tubes can still open voids - mention it to an inspector.

That works out to roughly 4 of Slope County's 1,215 square miles on soluble rock, 4 of them with the rock at or near the surface. With about 660 residents (1 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 372 map points about 1.8 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 how ground with no limestone still opens up.

Counties around Slope County

The nearest county rated sinkhole country is Fall River County, SD, about 223 miles south. 2 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 Upper and Middle Tertiary Rock, Undivided (sandstone), Pliocene to Miocene in age, covering about under 1% of the county.

The county also lies over the Williston Basin, a deep salt or gypsum basin. Deep beds rarely cause surface collapse on their own, but wells and mining into them have.

Against the rest of North Dakota

Slope County ranks 5 of 53 in North Dakota for soluble rock at the surface, and sits above 51% of US counties on that measure.