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Glaciation: The Ice Age

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.

  Appalachian Plateau Province
  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.  

  Glacial Grooves    

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.

Glacial Striations
 

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

 
Extent of recent glaciations in Pennsylvania       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).

Extent of Glacial Advances in Northeastern Pennsylvania
 
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. "Beaded Valleys" of Till Knobs
 
Transverse valley showing till shadow

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.

Glacial Lake Great Bend
  New Milford Sluiceway
 

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).

   
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