Sinkhole risk in Montgomery County, PA: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock: 5%
At or near the surface: 5%
Buried under cover: 0%

What to do about a house in Montgomery County

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 24 of Montgomery County's 483 square miles on soluble rock, 24 of them with the rock at or near the surface. With about 879,190 residents (1,821 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 427 map points about 1.1 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around Montgomery County

The nearest county rated sinkhole country is Union County, PA, about 103 miles northwest. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Bucks County, PA 16 mi northeast · 2% at surface
Philadelphia County, PA 19 mi southeast · 0% at surface
Delaware County, PA 20 mi south · 0% at surface
Chester County, PA 26 mi southwest · 7% at surface
Lehigh County, PA 30 mi north · 29% at surface
Berks County, PA 33 mi northwest · 22% at surface

What Montgomery County is built on

The largest mapped soluble unit here is the Conestoga Formation (limestone), Ordovician and Cambrian in age, covering about 2% of the county. Next is the Ledger Formation at 2%.

Against the rest of Pennsylvania

Montgomery County ranks 39 of 67 in Pennsylvania for soluble rock at the surface, and sits above 63% of US counties on that measure.