Wyoming County, Hop Bottom and Factoryville
Quadrangles
Locally known as the Nicholson Bridge, the Tunkhannock Viaduct
was named for the stream and valley it crosses.
In the long run, the great expense of building this structure
was considered to be less than a proposed route in the Carbondale/Forest City
area to the east. Here the railway contended with a climb of about 1200
feet as compared to about 2000 feet for the alternative. By taking
advantage of the canyon carved by the New Milford Sluiceway, considerable energy
savings could be realized by the frequent coal trains headed north.
The geology of the area of the Tunkhannock Viaduct is well-known, at least
partly because of surveys and studies conducted in association with the
construction of this bridge, which at that time was the largest reinforced
concrete structure of its kind. Put into service by the Delaware,
Lackawanna & Western Railroad in 1915, it is 2375 feet long and soars 240 feet
above the East Branch of Tunkhannock Creek.
At the site of the bridge, the valley is approximately 1300 feet
wide but is drained by a stream that is about 100 feet wide under normal flow
conditions. This suggests that the stream is underfit and once had a much
larger discharge (during glacial times) than it does currently. The
elevation of nearby hills on the sides of the valley is about 1300 feet, 600
feet higher than stream level.
Just south of the bridge site (and obscured by trees in the
photo above) is the confluence of Martins (Martens) Creek, which drains areas to
the north.
Bedrock, well-exposed along US Route 11 south of
Nicholson, is of Late Devonian age, a part of the Catskill Formation.
It is composed of interbedded sandstones, shales, siltstones and
mudstones. Several flagstone quarries are found in proximity.
Surficial deposits in the area are Late Wisconsinan and
consist of silty glacial till (Qwt) in hills and mixed outwash and
ice-contact deposits (Qwoic) in the stream valleys. In the area of the
bridge, deposits range from 50 to 100 feet in depth. The stream here
is cutting through an alluvial terrace bordered intermittently by 20-30
foot outwash and kame terraces both north and south of the bridge.
Excavations for the supporting piers were dug until
reaching bedrock. At the sides of the valley this was accomplished
in just a few feet. Most of the middle excavations had to dig
through about 60 feet of deposits, but one extended to 92 feet below
stream level.
Click on the figure above for a
larger-format graphic.
Reference:
Inners, J. D.,
and G. M. Fleeger, eds., From Tunkhannock to Starrucca:bluestone, glacial lakes, and great bridges in the "Endless Mountains" of
northeastern Pennsylvania:Guidebook, 67th Annual Field Conference of
Pennsylvania Geologists, Tunkhannock, PA.
145 pp.
