montgomery county today
47
FIGURE 41:
Geology
Council Rock in Lorimer Park is the county’s most
well-known geologic site.
Triassic shales characterize much of the western
portion of the county.
The rock bluffs along the Schuylkill River in Upper
Providence Township provide some great views.
Description of Local Geology
The two geologic regions of the county can be easily seen on the geology map. To the south of the Pennsylvania
Turnpike, I–276, is the Piedmont Upland Region. This area is characterized by several types of older igneous and
metamorphic rocks as well as a band of carbonate rock immediately south of the Turnpike. North of the
Pennsylvania Turnpike are broad bands of rock formations that make up the Triassic Lowlands. Most of these
sedimentary rocks are red shale and sandstone. Igneous diabase intrusions surround the Green Lane Reservoir area
and are found in a long ridge from Upper Pottsgrove to Spring Mountain.
¯
Pottstown
Norristown
Lansdale
Collegeville
Jenkintown
Conshohocken
476
276
76
476
t
u
422
t
u
202
611
100
309
Hatboro
Ambler
Pennsburg
73
663
29
29
73
Antietam Fm and Harpers Fm Undiv
Brunswick Fm
Bryn Mawr Fm
Chickies Fm
Conestoga Fm
Diabase
Elbrook Fm
Felsic Gneiss, Pyroxene bearing
Granite Gneiss and Granite
Ledger Fm
Limestone Fanoglomerate
Lockatong Fm
Mafic Gneiss, Hornblende bearing
Mafic Gneiss, Pyroxene bearing
Metadiabase
Patapsco Fm
Pensauken and Bridgeton Fms Undiv
Quartz Fanoglomerate
Serpentine
Stockton Fm
Wissahickon Fm (Albite-Chlorite Schist)
Wissahickon Fm (Oligioclase Mica Schist)
Source: PA Geological Survey
