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Introduction

Bedrock Geology of the Watershed Area

(Most graphics can be enlarged by clicking on them.)

Location of Tunkhannock Creek Watershed in NE PA

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 unconsolidated materials 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 Formation (Spechty-Kopf), 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 years ago.

Click for a large format, high quality geologic map of PA

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 alluvial plain.

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.