by Edward Cotter, Bucknell University
If you were flying over central Pennsylvania you would see long, parallel, linear ridges curving across the region (Figure 1) and between them fertile pastoral valleys with a quilt-like array of small farms (Figure 2). What could have caused this unique valley and ridge landscape pattern?
A look into the rocks underlying the landscape reveals a thick sequence of sedimentary layers that had been deformed so that the strata are no longer horizontal but are warped and folded. Ridges are made of hard, difficult-to-erode sandstones, while the valleys are sites of easier-to-erode limestones and shales.
So, to understand why central Pennsylvania has these unique features, we have to explore three chapters in its geologic history:
- the origin of the layered sedimentary sequence,
- the timing and cause of the folding and warping of these strata, and
- the millions of years of weathering and erosion that produced the present landscape.
The geology of central Pennsylvania can be understood in a context of moving tectonic plates, different types of continental margin, and changing directions of plate movement. This involves the powerful concepts known as Plate Tectonics. Before continuing, you might want to review an introduction to Plate Tectonics. Remember our objective is to explain and understand these three chapters in the geologic history. The explanation will be organized in terms of episodes:
The accumulation of the layered sedimentary sequence:
Episode 1: The sedimentary sequence began to form toward the end of the Precambrian as Pennsylvania was part of the passive trailing southeastern edge of ancestral North America, an intraplate continental margin of Atlantic-type.
Episode 2: Changing plate movement made eastern North America a tectonically active, Pacific-type margin, and central Pennsylvania received great quantities of additional sedimentary deposits largely from the erosion of the mountain ranges that were located along the continent's edge to the east.
Last Stage of Episode 2: Nearly final closure of the proto-Atlantic Ocean.
The folding and warping of that sequence:
Episode 3: As the plate convergence continued, finally the proto-Atlantic Ocean was closing completely as the continental mass of Africa collided with North America; this collision compressed, thrusted, and folded the previously deposited sedimentary sequence back toward the continent interior.
The long-term weathering and erosion:
Episode 4: Weathering and erosion of the folded and thrusted sedimentary rocks of Pennsylvania began when North America was part of the super-continent Pangaea, and continued as Pangaea rifted apart and the continents moved away from each other. Over time, difficult to erode strata, such as cemented sandstones, stood higher as ridges, while easier to erode strata, such as limestones and shales, became the sites of valleys.
|Introduction||Plate Tectonics||Episode 1||Episode 2||End of Episode 2||Episode 3||Episode 4|