Why "buried" limestone is not automatically safe

The question the numbers raise

Many county pages show most of the soluble rock as buried rather than at the surface, and it is natural to read that as good news. Sometimes it is. Often it is the opposite, because the most sudden sinkholes form where limestone sits under a layer of sand or clay, not where it is bare.

Three ways the ground gives way

Geologists describe three common types. A dissolution sinkhole forms where rock is at the surface and dissolves slowly into a bowl. A cover-subsidence sinkhole forms where the cover is sandy and sifts gradually into openings in the rock, so the ground sags over years. A cover-collapse sinkhole forms where the cover holds together, often clay, while a cavity grows underneath; the roof holds until it does not, and the hole opens in hours. That last type is the one in news footage, and it needs cover to exist.

Thin cover versus thick cover

The USGS map splits buried karst by how much lies on top. 351 counties have their main soluble unit under thin cover: less than 300 feet of ordinary sediment, or 50 feet or less of glacial till. SinkholeOdds treats that as live ground, and 186 of those counties land in one of the top two tiers. 342 counties have their rock under more than 50 feet of glacial deposits, mostly across the northern states, where the thick till damps surface collapse. Those are the "buried" counties that really are calmer.

What to do with that

If your county page shows buried rock, check which kind of cover it is in the "What this means" section. Under thin cover, treat the county as you would one with surface karst: look the parcel up on your state sinkhole map and ask an inspector about it. See how inspection works.

Where to go next

See how it varies: Izard County, AR, Southampton County, VA, Elmore County, AL.

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