Episode Four - Back to an Atlantic-type continental margin;
erosion of the deformed stratigraphic sequence

(about 270 million years ago to present)

Paleogeographic reconstruction of the continents during Late Jurassic time. North America has begun to rift apart from South America and Africa, creating the present-day Atlantic Ocean. From Dr. Ron Blakey's Global Earth History page, used with permission.

The result of this major period of tectonism was an Alpine-size mountain system with elevations up to 15,000 feet or so, and nearly 200 miles across, with its axis somewhere near the eastern border of Pennsylvania. Since there is no mountain system there at present, the next stage of geologic history is essentially nothing more than the gradual lowering of the mountainous terrain through operation of the forces of weathering and erosion. This erosion took place in the passive tectonic conditions characteristic of an Atlantic-type continental margin. Pangaea did not stay as an assembled supercontinent for very long. Beginning in Late Triassic time, the tectonic plates once again began to diverge to rift Pangaea apart (see Figure at right). As the Atlantic Ocean basin widened again, eastern North America resumed its passive ride on a westward-moving plate. Another Wilson Cycle was beginning.

While an enormous amount of erosion has taken place in the 270 million years since mountain-making, the rate at which that erosion took place has not been constant. For the first 50 million years or so erosion rates were accelerated in the near-equatorial conditions, and much of the elevation of the initial mountain system was lost. Erosion rates probably lessened as Pennsylvania drifted northward through arid subtropical latitudes, but in the Cretaceous Period, rates were again high as both global temperature and world-wide sea level were quite high. Through all this, isostatic rebound raised the regional elevation continually to make possible still more erosion.

Central Pennsylvania's characteristic linear valley and ridge landscape (above) is the consequence of this long-continued erosion. A close correlation exists between the elevation of particular rock units and their relative resistance to weathering and erosion (denudation). Well-cemented quartz sandstone is very resistant to denudation, so it is not surprising that the higher ridges consist of quartz-rich sandstones. On the other hand, limestones and shales can be eroded much faster, so once again it is not surprising that the valleys are underlain by limestones and shales. You will remember that the sedimentary sequence in central Pennsylvania consists of "packages" of strata that alternate between shallow-marine carbonate rocks (limestones and shales) and quartz-rich sandstones that were deposited in river systems.

Because those "packages" have been thrusted and folded into a particular configuration (see Figure immediately below) by the collision with Africa at the end of Episode 2, the elongate ridges are produced on top of several different sandstone-dominated rock "packages", while more than one "package" of limestones and shales underlie the intervening valleys (see photo below).

A NW - SE hypothetical cross section through central Pennsylvania, illustrating the folding and thrusting of sedimentary layers during the Permian. Modified from R. T. Faill in The Geology of Pennsylvania, 1999.

Kishacoquillas Valley as Viewed from Jack's Mountain. This topography is typical of the Valley and Ridge Province of central Pennsylvania.Valleys are typically underlain by easily-eroded underlain by shales and limestones, surrounded by ridges of highly-resistant sandstone.

One challenging question concerning the central Pennsylvania landscape has vexed geologists for more than a century. How did it come about that the major rivers, such as the Susquehanna, cut transversely across the ridges instead of flowing down the valleys? This issue is still being debated, but some elements of the answer appear to have a consensus. For most of the Paleozoic Era (see Episode 2), rivers in central Pennsylvania flowed westward away from easterly mountains toward the continent interior. In the early Mesozoic Era, the more intense denudation associated with equatorial climates significantly lowered those mountains. Then in the Late Triassic Period, the stretching and rifting apart of the crust associated with opening of the Atlantic made it possible for rivers in some of central Pennsylvania to carry sediment toward the southeast. Because their shorter routes to the sea were more efficient, gradually, over time the southeastward-directed rivers captured more and more of central Pennsylvania's drainage.





EROS-Landsat image of the Susquehanna River as it crosses the Valley and Ridge Province of central Pennsylvania. Confluence of the North and West Branch is visible at the top of the photo.


Introduction Plate Tectonics Episode 1 Episode 2 End of Episode 2 Episode 3 Episode 4

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