VALLEY FORGE NATIONAL PARK
Sedimentary rocks form as layer upon layer of sediment accumulate in various depositional environments. These layers, or strata, are probably the single most characteristic feature of sedimentary rocks.
The long outcrop on the south side of the Visitor Centers parking lot is the wall of an abandoned dolostone quarry. The layering produced during the formation of the rock is clearly visible. Although originally horizontal, the layers are now inclined toward the south at about 30 degree.
|The Chickies Quartzite Formation outcrops on the west side of Valley Creek directly upslope from the Upper Forge Site bridge. The quartzite is white on weathered surfaces and dark gray on fresh surfaces. The original quartz sand grains are easily visible. The layering of the rocks dips about 35 degrees to the south.|
|The Stockton Formation conglomerate dips moderately to the north. This small outcrop occurs along the jogging trail south and east of Star Redoubt.|
|The stone bridge over Valley Creek near Washington's Headquarters contains blocks of the Stockton sandstone which shows centimeter-scale layering.|
The Stockton Formation at Washingtons Headquarters has a distinctive form of bedding which is not uniform and horizontal like that of many sediments but is composed of many sets of interfering wedges of bedded material inclined at angles up to 35° from the horizontal. This form of bedding, called cross-bedding, is characteristic of sand dunes on land and of sand dunes formed by currents in rivers and under the sea.
Ripples in sediments are the low narrow ridges that are separated by wider troughs, both of which can be seen on the surfaces of windswept sand dunes, on underwater sandbars in shallow streams, and under the waves at beaches. They come in an assortment of shapes, sizes, and patterns that are characteristic of the wind or water currents that form them.
|Some of the building stone that makes up the sidewalk along the Valley Creek bridge contain ripple-marks. The shadows show that the ripples here are steeper on the slope toward the left of the page, that is, the downcurrent direction.|
Running water has the ability to sort sediments according to size and weight. Slow-moving water can only carry fine particles in suspension. Streams with moderate currents move sedimentary material along the bottom of the stream bed as well as in suspension. The faster the stream current, the greater the water's capacity to transport large sedimentary particles.
|The stone bridge over Valley Creek near Washington's Headquarters contains blocks of the Stockton conglomerate. This unit contains large sedimentary particles and is essentially a cemented gravel. The large particle size suggests deposition in a swiftly moving stream.|
Introduction Geologic History Geologic Map
Erosional Features Depositional Features
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