Sinkhole risk in Custer County, OK: Sinkhole country

Sinkhole country

Fast-dissolving gypsum or salt beds under a large part of the county.

On soluble rock: 44%
At or near the surface: 44%
Buried under cover: 0%

What to do about a house in Custer County

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

That works out to roughly 431 of Custer County's 989 square miles on soluble rock, 431 of them with the rock at or near the surface. With about 28,266 residents (29 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 404 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 the signs to look for and how an inspection works.

Counties around Custer County

The closest county with no soluble rock mapped is Hemphill County, TX, about 73 miles west. 6 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Dewey County, OK 23 mi north · 57% at surface
Washita County, OK 25 mi south · 64% at surface
Blaine County, OK 36 mi northeast · 55% at surface
Roger Mills County, OK 42 mi west · 31% at surface
Beckham County, OK 47 mi southwest · 32% at surface
Caddo County, OK 48 mi southeast · 79% at surface

The rock underneath

The largest mapped soluble unit here is the Rush Springs Formation (sandstone), Early Permian in age, covering about 27% of the county. Next is the Cloud Chief Formation at 17%.

44% of the county sits on gypsum or salt, which dissolves in years rather than millennia. That share matters more than its size suggests.

The county also lies over the Permian Basin, Anadarko Basin,Dalhart Basin, Palo Duro 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 Oklahoma

Custer County ranks 14 of 77 in Oklahoma for soluble rock at the surface, and sits above 85% of US counties on that measure.