All of the rocks in the park are of sedimentary origin. Sedimentary rock forms from the accumulation of weathered or dissolved particles from preexisting rocks.  These particles are then transported, deposited, and lithified.  The processes of erosion and deposition bring about the creation of sedimentary rocks. Erosion is the process by which rock is weathered and transported from its place of origin. Deposition is the process by which eroded particles are dropped from water, wind, or solution.

Erosional Features

Dissolving Rocks

In a temperate climate such as that in southeastern Pennsylvania, water is an effective agent of chemical weathering. Solution activities caused by the reaction of water with carbonate rock have created a geologically interesting and scenic topography to which scientists have assigned the name "karst". This word is derived from the Jugoslav "kars", which means stone, and was used to refer to the solution features in limestone which occur on a plateau in Jugoslavia and in adjacent parts of Italy, both of which border on the Adriatic Sea. "Karst" is a comprehensive topographic term applied to areas underlain by carbonate rocks which possess a topography peculiar to and dependent upon underground solution and the diversion of surface waters to underground routes. Karst areas characteristically exhibit an irregular terrain punctuated with many depressions, called sinkholes or sinks.

As acidic water dissolves limestone, cracks in the rock become larger, fissures and joints widen, and the surface of the ground becomes pitted because of the formation of sinks. All water that falls upon the ground in regions of limestone bedrock eventually finds its way into underground channels. In areas where solution of limestone is active, the terrain is characterized by sinking streams, sinkholes, caverns, resurgences, and other features attending underground.

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The parks southern lowlands are characterized by a hummocky topography peculiar to and dependent upon underground solution and the diversion of surface waters to underground routes.

Breaking Rocks

Mechanical weathering is the physical breaking of rock. This large pile of rock along Valley Creek gorge is called a talus slope. It formed by the mechanical weathering process called frost wedging in which water works its way into cracks or voids in rocks and, upon freezing, expands and wedges the rock apart. The talus slopes along Valley Creek gorge are composed of blocks of Chickies quartztite loosened by mechanical weathering.


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Running Water

Streams are the primary agent of erosion on the Earth’s surface. Flowing water has the capability to loosen and remove bedrock, thereby deepening its channel. The erosional capability of streams is dramatically illustrated by the Valley Creek gorge, which cuts through the resistant rocks underlying Mount Misery and Mount Joy.


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This picture of the Port Kennedy quarry located north of the park's Visitor Center depicts erosion that happened at least 200 million years ago. The light-colored rock exposed at the base of the quarry wall is the Ledger dolostone. Red sandstones and shales of the Stockton Formation crop out at the top of the quarry. There is an age difference of about 300 million years between these two rock types. What happened to the rock layers that were deposited throughout the Paleozoic Era? At some point before the deposition of the red beds occurred, older rocks were destroyed by some form of erosion or deformation. Another possibility is that no sediments were deposited during this time. A gap like this in the rock record is known as an unconformity.

Unconformities range from minor erosional breaks to strong angular discordances in bedding. The Cambrian-Triassic unformity at Valley Forge is considered an angular unconformity because the rock layers separated by the unconformity are not parallel. The layering in the Ledger dolostone is inclined steeply to the south while layering in the Stockton Formation dips gently to the north. The cartoon below depicts the development of an angular unconformity.


For more information about unconformities, see these web sites:

Geosciences Program at Emory University

Earth Science Department at University of Northern Colorodo


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Figure borrowed from

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Introduction           Geologic History            Geologic Map

Erosional Features           Depositional Features          

More Interesting Geologic Features          References