Sinkhole risk in Union County, PA: Sinkhole country

Sinkhole country

Soluble rock at or near the surface across much of the county.

On soluble rock: 66%
At or near the surface: 53%
Buried under cover: 13%

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 208 of Union County's 316 square miles on soluble rock, 167 of them with the rock at or near the surface. With about 42,159 residents (133 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 476 map points about 0.8 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: why the 13% under cover still counts, and the signs to look for and how an inspection works.

Counties around Union County

The closest county with no soluble rock mapped is Tioga County, PA, about 57 miles north. None of the 6 closest counties land in the same band, so Union County stands out from its surroundings.

Snyder County, PA 14 mi south · 38% at surface
Northumberland County, PA 20 mi southeast · 26% at surface
Montour County, PA 21 mi east · 40% at surface
Lycoming County, PA 26 mi north · 6% at surface
Columbia County, PA 34 mi east · 4% at surface
Juniata County, PA 35 mi southwest · 45% at surface

What Union County is built on

The largest mapped soluble unit here is the Clinton Group (shale), Silurian in age, covering about 20% of the county. Next is the Bloomsburg and Mifflintown Formations, undivided at 16%.

Against the rest of Pennsylvania

Union County ranks 1 of 67 in Pennsylvania for soluble rock at the surface, and sits above 88% of US counties on that measure.