Sinkhole risk in Armstrong County, PA: Pockets of it

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 239 of Armstrong County's 653 square miles on soluble rock, 239 of them with the rock at or near the surface. With about 63,679 residents (97 per square mile), homes are spread across towns and farmland, and a mapped unit may or may not reach the developed parts. The figures come from 609 map points about 1.0 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 Armstrong County

The nearest county rated sinkhole country is Washington County, MD, about 121 miles southeast. 6 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

• Indiana County, PA 23 mi southeast · 16% at surface
• Butler County, PA 25 mi west · 35% at surface
• Clarion County, PA 27 mi north · 36% at surface
• Jefferson County, PA 33 mi northeast · 24% at surface
• Westmoreland County, PA 35 mi south · 26% at surface
• Allegheny County, PA 36 mi southwest · 28% at surface

What Armstrong County is built on

The largest mapped soluble unit here is the Allegheny Formation (sandstone), Pennsylvanian in age, covering about 36% of the county. Next is the Monongahela Group at under 1%.

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

Armstrong County ranks 10 of 67 in Pennsylvania for soluble rock at the surface, and sits above 82% of US counties on that measure.

• Union County 53%
• Cumberland County 48%
• Lancaster County 46%
• Juniata County 45%
• Mifflin County 45%