Geoscience Reference
In-Depth Information
drains into the Atlantic Ocean. The SRB extends from the Atlantic coastal
plain inland approximately 260 km, crossing several physiographic regions
(Fig. 9.1) of the Appalachian Highlands. These regions generally run northeast-
southwest, approximately parallel to the Atlantic coastline. The physiographic
provinces covered by the basin include Appalachian Plateau, Valley and Ridge,
Blue Ridge and Piedmont (Marsh and Peirce, 1995; Pennsylvania Department
of Environmental Protection, 2009).
The northern and western portions of the basin are located in the
Appalachian Plateau. This region is characterized by sedimentary rocks of
Devonian, Mississippian and Pennsylvanian age that are flat lying or very
gently folded. The rocks are dominantly siliciclastics. The Pennsylvanian
formations contain coal beds that are widely mined. The streams generally
flow in deep, steep-sided valleys with 150 to 300 m of local relief. Dissection
of the plateaus is variable, with some portions having extensive summit areas
while other portions are nearly all in slope. The Appalachian Plateau is largely
forested. The population is concentrated in widely scattered towns with a
population of a few hundreds to a few thousands. The Valley and Ridge
section occupies the central portion of the basin. This region is characterized
by strongly folded and faulted sedimentary rocks of Paleozoic age. Erosion of
this terrain has produced even-crested parallel ridges of Paleozoic sandstone
with intervening valleys formed on less resistant rock types. The ridges typically
have 300-450 m of local relief. The valleys are generally hilly. The ridges are
primarily forested while the valleys are primarily comprised of agricultural
lands. The population is concentrated in towns of a few hundred to several
thousand people. The larger streams flow parallel to the ridges, while the
trunk streams cross the ridges in deeply cut water gaps. The Susquehanna
River and its major tributaries actually flow across this structural and
topographic grain. The Blue Ridge section is a broad anticlinorium of largely
Lower Cambrian metasediments with a core of late pre-Cambrian igneous and
meta-volcanic rocks. The Blue Ridge is about 10 km in width, and ends
abruptly about 24 km west of the Susquehanna River. Summit areas are gently
rounded, with extensive flat areas. The Piedmont section is located seaward of
the Blue Ridge. It is approximately 60 km wide where it crosses the SRB.
Landuse is a mix of forestry and agriculture. The Piedmont consists largely of
uplands underlain by meta-sedimentary and meta-volcanic rocks, with minor
lowlands underlain by Mesozoic age sediments and igneous intrusions. The
structural grain of the Piedmont is parallel to that in the neighbouring
physiographic sections: northeast-southwest. Local relief is typically 30-90
m. The Susquehanna River flows through a rugged bedrock valley as it crosses
the Piedmont.
The climate of the SRB is humid continental, and reflects both an
alternation and interplay between oceanic and continental air masses, and
their associated weather. The Appalachian Plateau section of the SRB is
characterized by average annual precipitation in the range of 90-100 cm,
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