Springs generally crop up in Valley Forge National Park at the contact between the Stockton Formation and the Ledger Dolostone. The shales, sandstones, and conglomerates of the Stockton Formation are very porous. That is, they have many void spaces in which water can be stored. In addition, there are many connections between the pore spaces so the rock is permeable. Groundwater is easily stored and transmitted through the Stockton Formation and thus, it is a local aquifer (groundwater reservoir). On the contrary, the massive and fine-grained Ledger Dolostone has a low porosity and low permeability. It acts as a barrier to groundwater flow and is thus called an aquiclude.


This springhouse, built in colonial times, takes advantage of the water that naturally flows from the ground at the contact between the Stockton Formation sandstones (upper unit) and the Ledger dolostone (lower unit).


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spring.jpg (71975 bytes) A schematic drawing of a contact spring.


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The warping of the stone wall outside of Washington’s Headquarters is the result of a type of erosion called creep. Creep involves the gradual downhill movement of soil and unconsolidated material. One of the primary causes of creep is the alternate expansion and contraction of surface material caused by freezing and thawing or wetting and drying. Creep may also be initiated if the ground becomes saturated with water. Following a heavy rain or snowmelt, a waterlogged soil may lose its internal cohesion, allowing gravity to pull material downslope. Although the movement is imperceptibly slow, its effects are quite obvious.

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