Download monroe border fault, bucks county

OUTSTANDING GEOLOGIC FEATURES OF PENNSYLVANIA
MONROE BORDER FAULT
BUCKS COUNTY
Location
Route 611 and Rehnenberg Rd., Bucks Co., Durham Twp., lat:
40.5723, lon: -75.1925, Riegelsville 7.5-minute quadrangle
The site has been designated a National Natural
Landmark. Above, view of Triassic rocks dipping to
the northwest and plaque, which reads “This site
possesses exceptional value as an illustration of the
nation’s natural heritage and contributes to a better
understanding of man’s environment.” Right, a
geologic cross section looking east.
Geology
The Monroe Border Fault (Triassic Border Fault), and the
adjacent rocks, together provide a textbook example of
Pennsylvania’s long geologic history. The area has undergone
multiple episodes of deposition, mountain building, and erosion.
Immediately north of the border fault are tightly folded and
faulted rocks, which are from a series of complexly-folded thrust
sheets pushed sideways about 440 to 500 million years ago. This
is the Reading Prong section of the New England physiographic
province. It is composed of metamorphosed sedimentary rocks of
dolomite and quartzite that are over 500 million years old, and
folded-in Precambrian gneisses (or granitic rocks) that are some
of the oldest rocks in the state at over a billion years old. Each
unit had its own geologic history before being pushed over
Cambrian-age limestone. After the Alleghanian mountainbuilding event (about 290 million years ago), a down-warped
basin formed in fairly high terrain as Africa began its separation
from North America about 250 million years ago. Sediment
poured in during the Triassic Period as the basin deepened into
what is now the Gettysburg-Newark Lowland physiographic
section. Coarse-grained sediments accumulated along the basin
edges. Rivers laden with sediment built alluvial fans. The climate
was generally warm but variable; dinosaurs passed through the
region while lakes came and went. About 200 million years ago,
diabase sills and dikes intruded the basin. Normal faulting and
some folding then developed as the basin tilted to the northwest.
The border fault marks the boundary between these geologic
remnants. Millions of years of erosion have unearthed evidence
of its deep geologic history.
Recommended Reading
Inners, J. D., 1980, Nockamixon State Park, Bucks County—Rocks and joints: Pennsylvania Geological Survey, 4th ser.,
Park Guide 14.
For information regarding outstanding geologic features, contact the Pennsylvania Geological Survey:
Bureau of Topographic and Geologic Survey, 3240 Schoolhouse Rd., Middletown, PA 17057
717-702-2017 www.dcnr.state.pa.us/topogeo
Nockamixon State Park is located just five miles south.
S. Reese, 2013