If you live in Berks, Lehigh, Northampton, Bucks, Montgomery, or Chester counties — you live on or near the Reading Prong, one of the most uranium-rich rock formations in the eastern United States. The Reading Prong is the geological reason eastern Pennsylvania has some of the highest indoor radon readings in America, and it's the formation directly responsible for the 1984 Watras 2,700 pCi/L record-setting reading.
What is the Reading Prong?
The Reading Prong is a band of Precambrian-age (over 1 billion years old) metamorphic and igneous rock — primarily gneiss and granite — that extends from Reading, Pennsylvania northeast through the Lehigh Valley into northern New Jersey and southern New York. "Prong" is a geological term referring to a finger of older rock projecting through younger overlying rocks; in this case, billion-year-old Precambrian basement rock that surfaces in a long, narrow band through eastern PA.
The Reading Prong is part of the broader Grenville Province — a continent-scale belt of Precambrian rock that runs across eastern North America from Canada south to Mexico. The PA portion of the Grenville Province is unusually uranium-rich.
Why Reading Prong rock contains so much uranium
Reading Prong gneiss and granite formed during a Precambrian mountain-building event (the Grenville Orogeny) approximately 1.1 billion years ago. The melting and recrystallization process concentrated uranium in specific mineral phases — primarily zircon, monazite, allanite, and some uranium-rich accessory minerals — within the gneiss and granite matrix.
Per the Pennsylvania Geological Survey (PA DCNR Bureau of Topographic and Geologic Survey), Reading Prong rock uranium concentrations average 5-15 parts per million across most of the formation — roughly 3-6× the typical continental crust baseline. Local concentrations in specific gneiss bodies can reach 50+ ppm.
Uranium-238 decays through a chain that produces radon-222 — the form of radon that reaches indoor air. Higher uranium content in the bedrock means continuous radon generation from the rock for billions of years.
The Reading Prong counties in Pennsylvania
The Reading Prong is most extensively exposed in:
- Berks County — including Reading, Pottstown, Boyertown (the Watras Incident location), and surrounding municipalities. Berks is the namesake county.
- Lehigh County — including Allentown, Bethlehem (southern part), Emmaus, and the Lehigh Valley.
- Northampton County — including Easton, Bethlehem (northern part), Nazareth, and Bath.
- Bucks County — including Quakertown, Perkasie, and northern Bucks municipalities. The Reading Prong runs through northern Bucks but not the southern Philadelphia-suburb portion.
- Montgomery County — northern Montgomery municipalities including Pottstown (Berks County boundary), Souderton, and Boyertown adjacent areas.
- Chester County — northern Chester County including Phoenixville and surrounding areas.
Adjacent counties — Lancaster, Schuylkill, Carbon — have variable exposure to the Reading Prong but are not the geological core.
How Reading Prong radon enters homes
Three transport mechanisms make Reading Prong indoor radon especially severe:
- Thin soil overburden. Much of the Reading Prong has shallow soil over bedrock — sometimes only a few feet. Radon generated in the bedrock reaches the soil surface (and home foundations) quickly.
- Fractured bedrock. Reading Prong gneiss has been fractured by hundreds of millions of years of tectonic stress. These fractures act as gas transport pathways from deep rock to the surface.
- Direct foundation contact. Many Reading Prong county homes are built directly on or with minimal soil above the bedrock. Sub-slab depressurization in these homes pulls gas directly from the uranium-rich rock itself, not from intervening soil.
The Watras Incident in geological context
The 1984 Watras home in Boyertown sat directly on Reading Prong gneiss with very thin soil overburden. The combination — uranium-rich bedrock + thin soil + a finished basement used as living space + foundation cracks providing efficient gas transport — produced the 2,700 pCi/L reading that triggered the federal EPA radon program.
For the full Watras story and its consequences for the modern radon industry, see The 1984 Watras Incident: How Boyertown Discovered America's Radon Problem.
Three other Pennsylvania radon drivers (briefly)
The Reading Prong is Pennsylvania's most famous radon driver, but the state has three distinct geological radon sources:
- Reading Prong (eastern PA) — uranium-bearing Precambrian gneiss and granite (this article).
- Appalachian Plateau Devonian shale (western PA) — uranium-bearing Marcellus, Burket, and Chattanooga shale formations underlying Pittsburgh Metro and western PA. Less famous than the Reading Prong but a major radon driver in Allegheny, Washington, and Westmoreland counties.
- Cambrian-Ordovician carbonate karst (south central PA) — limestone and dolomite formations with karst conduit transport across Lancaster, Lebanon, Dauphin (Harrisburg), Cumberland, and Centre (State College) counties. Similar geology to Kentucky's Inner Bluegrass karst belt.
What this means for Reading Prong county homeowners
The geological radon source is permanent — Reading Prong gneiss has been generating radon for over 1 billion years and will continue indefinitely. The Reading Prong county baseline expectation should be: test, expect elevated, mitigate.
Use our Pennsylvania radon calculator to map your test result against EPA guidance and estimate mitigation cost. For Berks, Lehigh, Northampton, Bucks, Montgomery, or Chester County homes, the calculator's regional cost multipliers (1.05-1.15× for major metros, lower for rural townships) reflect typical Reading Prong contractor pricing.
For mitigation, work with an NRPP + PA DEP-certified contractor per Title 25 Pa. Code Chapter 240. Reading Prong county contractors typically install Active Sub-Slab Depressurization (ASD) systems with diagnostic pressure-field extension (PFE) testing pre-install — see the PA Radon Certification page for details on what PFE testing is and why it matters in high-radon homes.
For Pennsylvania real estate transaction implications, see Pennsylvania Real Estate Seller Disclosure Law (68 Pa. C.S. §§ 7301-7315) Explained.
Sources: Pennsylvania Geological Survey (PA DCNR Bureau of Topographic and Geologic Survey), USGS publications on the Reading Prong + Grenville Province, EPA Map of Radon Zones, PA DEP Radon Division.