(Most graphics can be enlarged by clicking on
The bedrock of the Tunkhannock
Creek watershed area is described elsewhere in this Atlas by Cook (2002) in
conjunction with an evaluation of groundwater resources.A summary including graphics and concepts is included here.
The geology of the Tunkhannock
Creek Watershed area is based on its bedrock and the unconsolidatedmaterials that are atop it.
While other parts of the state have a complex assemblage of
rocks exposed at the surface (right), our bedrock (the solid
rock that forms the foundation of the area) is almost
entirely of the Catskill Formation, which is aged to the Late
Devonian, about 380 million years ago.
(There are frequent references to the geologic time scale throughout this
website.To refresh yourself, please view this chart.)In the southernmost part of the watershed, some of the highest hills are
capped with the remnants (monadnocks) of younger rocks, of the Huntley Mountain
which is aged to the Devonian-Mississippian
boundary, about 350 million years old.These
bedrock hills are ‘islands’ rising out of a ‘sea’ of glacial till,
sediments left behind by the glaciers which last left the area about 20,000
Sevon (1985) found
most of the Catskill Formation rocks in the area to be non-marine, and to have
been deposited by meandering or braided streams on either a delta plain or
Why are there no younger rocks in our area?
After all, they exist in other parts of the state.
The answer lies in the competing processes of rock formation and erosion.Evidence from elsewhere tells us that younger sedimentary rocks were
formed atop the rocks we now see at the surface.However, this deposition took place during the time when
Northeastern Pennsylvania was under water.(See the Historical Geology section).Thick layers of sediment being shed from the mountains to the east were
compressed, compacted, and sedimented into solid rock.Subsequently, the area was uplifted, and the rocks immediately were
subjected to the forces of erosion. For
hundreds of millions of years, we have been in an erosional era. In
other words, all the younger rocks have been eroded away, just as the Devonian
rocks at the surface are being eroded today.
The highest elevation in the watershed is Elk Mountain at
2680 feet. The lowest elevation is 603 feet at the
confluence of Tunkhannock Creek with the Susquehanna River
in Tunkhannock. Thus, the watershed relief is 2077
feet. As indicated by the topographic map to the
right, generally the highest elevations (indicated by darker
colors) in the watershed are in the northeast, while the
lowest elevations (lighter colors) are in the southwest.
The landscape of the watershed is dissected by an abundance of streams, all of
which are ultimately part of the Susquehanna River
drainage system, the largest in the state. Most of the area streams
exhibit a dendritic drainage pattern, which typically develops when water flows
over a mostly uniform surface, and when no other factors play a major role in
determining their course. Cross-sectional profiles (below, left) also
indicate the dissected nature of the landscape.
However, much of the northwestern two-thirds of the watershed lies in the Susquehanna
County Parallel Drainage section where the drainage is surprisingly linear
and parallel due to to jointing patterns in the underlying bedrock.