Sinkhole risk in Allegheny County, PA: Pockets of it

Pockets of it

Soluble rock at the surface or under thin cover in part of the county.

On soluble rock: 28%
At or near the surface: 28%
Buried under cover: 0%

The next step for one property

Check the parcel against your state survey sinkhole map, and ask for an inspection if cracks, depressions or sticking doors show up.

That works out to roughly 203 of Allegheny County's 730 square miles on soluble rock, 203 of them with the rock at or near the surface. With about 1,231,814 residents (1,688 per square mile), most of the county is built up, so a mapped unit is likely to sit under somebody's street. The figures come from 255 map points about 1.7 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 Allegheny County

The nearest county rated sinkhole country is Washington County, MD, about 129 miles southeast. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Washington County, PA 23 mi southwest · 31% at surface
Beaver County, PA 24 mi northwest · 9% at surface
Westmoreland County, PA 29 mi east · 26% at surface
Butler County, PA 31 mi north · 35% at surface
Hancock County, WV 31 mi west · 2% at surface
Brooke County, WV 34 mi southwest · 38% at surface

What Allegheny County is built on

The largest mapped soluble unit here is the Monongahela Group (limestone), Pennsylvanian in age, covering about 28% of the county.

The county also lies over the Michigan Basin; Ohio Basin; Appalachian 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 Pennsylvania

Allegheny County ranks 20 of 67 in Pennsylvania for soluble rock at the surface, and sits above 78% of US counties on that measure.