Areas which have similar landscape, geology, and
subsequently biota, are called physiographic provinces. Pennsylvania is divided into seven distinct
provinces.
Much of Northeastern Pennsylvania is located in the Appalachian
Plateau province, which covers the largest area in the state. Tunkhannock
Creek drains areas of the Glaciated Low Plateau region. The plateau has
long been affected by the erosive power of its streams and rivers, which have
cut deep valleys and left the hilly topography which is so familiar to us.
Why does the land of the watershed look as
it does? The landforms and surficial geology of the Tunkhannock Creek watershed are reflections of the
bedrock and unconsolidated materials laying atop it.
Exposed bedrock in the watershed area is almost entirely of Late
Devonian age and
is known as the Catskill Formation of sandstone and shale. However, much of what is apparent to the eye, the surficial geology, is a result of
the action of glaciers.
How do we know that glaciers were here? That they were the cause of
much change and alteration of the landscape? After all, there are none here
today! In a sense, geologists act much the same way as a forensic
specialist. When called to view the scene of a crime, the deed is
done. The perpetrator is long gone. How do we know that a crime was
committed? By a trail of evidence and clues left behind.
In a similar way, glaciers leave behind a trail of
evidence which is unmistakable and better than any murderer! These
include rounded hills, valleys choked with glacial deposits, U-shaped
valleys, moraines, sand and gravel deposits, bogs, etc.
Comparison of landforms such as these in our
area can be made to the actions of active glaciers now at work in
Alaska,Greenland, and Antarctica. When landforms (such as the
photos taken in other areas of remarkable
striations and grooves
in bedrock) found in our area are compared to
those which are the result of advancing and retreating ice in these active
areas, it is obvious that our area, too, was sculpted by ice. During the last
one or two million years, northeastern Pennsylvania has been affected
by a series of episodes
of alternating glacial and interglacial periods. There
have been at least four glacial advances across the Tunkhannock Creek Watershed
area in that time.
Each succeeding advance usually obliterates most of the
evidence of its predecessors. The map indicates that at least two advances
moved even farther south than the most recent, the Wisconsinan. The most
southerly limit is that of the Pre-Illinoian (formerly known as Kansan), which
occurred more than 850,000 years ago. More recently, a late Illinoian
advance about 150,000 years ago extended approximately 10 miles beyond the Wisconsinan. The ice sheet advanced and retreated several times during the
Early and Middle Wisconsinan reaching into New York State, but only the Late Wisconsinan advance reached our area.
Striations from earlier glaciations are
oriented similarly to those of Wisconsinan age. Older glacial frontal
boundaries nearly parallel the most recent. These findings suggest that
older glaciations entered and retreated from the state in a similar manner and
probably would have had similar erosional and depositional actions.
What do you think Pennsylvania might have looked like
during these glaciations? Here's one interpretation.
At the point of farthest glacial advance, the ice in the area
of the town of Tunkhannock was probably
about 3,000 feet thick, ranging to about 5,000 feet at the PA-NY border. How do we know that? This estimate is based
on profiles of existing ice sheets in other areas of the world. We find
that in order to cause the kinds of alterations that we see in our landscape,
usually this thickness of ice is required. Additionally, estimates of the
amount of isostatic rebound due to
the removal of the weight of the ice indicates rebound of about 100 feet in this
area (based on rebound in the area of the Great Lakes).
It is estimated that the total duration of ice cover
in the northern tier counties of Pennsylvania was about 6000 years
and about 1000 years at the terminus. Ice last retreated from
our area about 20,000 years ago, melting at the rate of about 30
years per mile (assuming no readvances). Hence, it took approximately 1000 years to recede from
Tunkhannock to the New York border (animation by Illinois State Museum GIS Lab).
However, field evidence suggests that recession was not wholly continuous, with
the terminus hesitating for perhaps several decades in numerous
areas. While the front of the ice stood still, the 'conveyor belt' motion
of the ice surface would have continued to deposit materials. Each
hesitation would have led to a thicker deposition layer, resulting in till knobs,
piles of glacial debris, which would have blocked valleys. As Braun (2002) suggests, "A series
of such knobs formed 'beaded valleys' that have a series of wider and narrower
segments." In the graphic to the right (modified from Braun),
tan areas represent bedrock outcroppings; blue represents ponds and/or wetlands;
and much, if not all, of the remaining areas indicating till deposits of varying
thicknesses.
However, preglacial valleys transverse (perpendicular) to ice flow show an asymmetry
with south and southwestern facing sides gently sloping under a thick till
shadow, while north facing sides are steeper usually with exposed bedrock.
As shown in the graphic to the left (modified from Braun 2002), the postglacial
stream often cut a bedrock gorge on the south side, working its way down
vertically.
As the glacier retreated and while it was close by
to the north, perhaps thousands of years of severe periglacial climatic
conditions ensued. Mechanical weathering driven by freeze-thaw cycles
would have helped to break up exposed outcrops. Cold conditions prevailed,
much as we see today in the northern tundra and at the edge of glaciers.
It was not until about 13,000 years before the present that forests covered the
area.
As the ice retreated to the north, meltwaters flowed down south-trending
valleys. The ice passed over a high ridge, an east-west regional
divide, located just south of New Milford in Susquehanna County.
Thereafter, drainage waters were impounded in north-draining valleys forming a
series of post-glacial lakes. As the ice further retreated north of Great
Bend in Susquehanna County, these lakes joined to become Glacial Lake Great Bend
(right, modified from Braun, 2002), which extended along the glacial front for
about 25 miles.
The lake's outlet was to the south, through the "New
Milford Sluiceway" to Martins Creek, meeting Tunkhannock Creek near
Nicholson (Harrison, 1966).
Through several glaciations, a 600-foot notch was carved through the divide by
the meltwaters.
Although the glacier is long-gone, it has left behind a legacy
of deposits, some of which have economic value. In scattered areas of the
watershed there are ice-contact deposits, kame deltas, and eskers, as well as outwash deposits, which can be and
are mined for sand, gravel, and road aggregate materials.
As streams cut through these deposits, they
are sometimes mobilized and carried to areas far beyond the extent of ice cover,
as evidenced in the map of glacial deposits (left).
