Sinkhole risk in Fall River County, SD: Sinkhole country

The next step for one property

Get a sinkhole or geotechnical inspection before buying, building or adding weight to a foundation.

That works out to roughly 642 of Fall River County's 1,740 square miles on soluble rock, 642 of them with the rock at or near the surface. With about 7,359 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 390 map points about 2.1 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: why gypsum and salt move faster than limestone, and the signs to look for and how an inspection works.

Counties around Fall River County

The closest county with no soluble rock mapped is Box Butte County, NE, about 73 miles south. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

• Custer County, SD 32 mi north · 30% at surface
• Dawes County, NE 40 mi southeast · 33% at surface
• Oglala Lakota County, SD 48 mi east · 4% at surface
• Niobrara County, WY 49 mi west · 15% at surface
• Sioux County, NE 53 mi south · 13% at surface
• Pennington County, SD 64 mi northeast · 13% at surface

Which formation it is

The largest mapped soluble unit here is the Pierre Shale (shale), Late Cretaceous in age, covering about 26% of the county. Next is the Niobrara Formation at 4%.

28% 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

Fall River County ranks 2 of 66 in South Dakota for soluble rock at the surface, and sits above 82% of US counties on that measure.

• Lawrence County 42%
• Custer County 30%
• Gregory County 17%
• Pennington County 13%
• Butte County 6%