Sinkhole risk in Harper County, OK: Sinkhole country

Sinkhole country

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

On soluble rock: 61%
At or near the surface: 61%
Buried under cover: 0%

What this means before you buy

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

That works out to roughly 631 of Harper County's 1,039 square miles on soluble rock, 631 of them with the rock at or near the surface. With about 3,209 residents (3 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 370 map points about 1.7 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Harper County

The closest county with no soluble rock mapped is Hemphill County, TX, about 76 miles southwest. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Clark County, KS 31 mi north · 37% at surface
Woodward County, OK 34 mi southeast · 46% at surface
Comanche County, KS 35 mi northeast · 39% at surface
Ellis County, OK 40 mi south · 14% at surface
Woods County, OK 45 mi east · 42% at surface
Beaver County, OK 46 mi west · 5% at surface

Which formation it is

The largest mapped soluble unit here is the Marlow Formation (sandstone), Early Permian in age, covering about 27% of the county. Next is the Rush Springs Formation at 15%.

61% 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

Harper County ranks 8 of 77 in Oklahoma for soluble rock at the surface, and sits above 90% of US counties on that measure.