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Site content related to keyword: "Wissahickon Formation"

Groundwater Station: DGS Well Bc43-01

DGS Well Bc43-01

Station Type: 
Groundwater
Period of Record: 
1974 to present
Frequency: 
Quarterly
Map County: 
New Castle County
Map Location: 
39.781700, -75.625099

Geochemical Data of Mafic Rocks in Delaware Piedmont, PA and MD

Geochemical Data of Mafic Rocks in Delaware Piedmont, PA and MD

Geochemical data from Ordovician and Silurian mafic rocks in the Wilmington Complex in Delaware, the James Run Formation in Cecil County, Maryland, and the Wissahickon Formation in Delaware and Pennsylvania were collected in conjunction with preparation of a new geologic map of the Delaware-Pennsylvania Piedmont. Although concentrations of most elements may have been disrupted by metamorphism, the more stable high field strength elements, including the rare earth elements (REE), are consistent within mapped lithodemic units and are compared to modern basaltic magmas from relatively well known tectonomagmatic environments.

Our results are similar to those for other Appalachian mafic rocks and suggest a suprasubduction zone tectonic setting for the Wilmington Complex and the James Run Formation in Cecil County, Maryland. Thus, the rocks of the Wilmington Complex plus the James Run Formation in Cecil County may be stages in a continuum that records the temporal magmatic evolution of an arc complex.

Outcrop Cc12-c: The Red Clay Creek Edge

Rock Outcrop Cc12-c: The Red Clay Creek Edge

Along the edge of the Red Clay Creek exists a large outcrop that extends out into the stream. This rock is part of the Wissahickon Formation, with pelitic facies, ½" elongated sillimanite nodules, and disharmonic folds. The compositional layering of this rock is 1/8 – ½" of biotite rich layers alternated with fine-grained psammitic layers (not quartz-feldspar layers). Some of these layers are sheared (shear zones). The sillimanite nodules, pegmatite pods, and shear zones in this rock are all parallel to fold axes. The axial plane of these folds is 20 degrees east of north, plunges 42 degrees northeast, and dips 90 degrees. Within this large outcrop are several 2-3' layers of “rock that rings” (when hit) and are folded with petitic gneiss. This pelitic gneiss shows more intense folding while the rest of the rock is gently folded. The “rock that rings” is also peppered with small lavender garnets.

Geologic History of the Delaware Piedmont

Fig A. Cross section of eastern North America as it may have looked 543 million years ago, active volcano is offshore.

The Delaware Piedmont is but a small part of the Appalachian Mountain system that extends from Georgia to Newfoundland. This mountain system is the result of tectonic activity that took place during the Paleozoic era, between 543 and 245 million years ago. Since that time, the mountains have been continuously eroding, and their deep roots slowly rising in compensation as the overlying rocks are removed. It is surprising to find that although the Delaware Piedmont has passed through the whole series of tectonic events that formed the Appalachians, the mineralogy and structures preserved in Delaware were formed by the early event that occurred between 470 and 440 million years ago, called the Taconic orogeny.

Outcrop Bb25-c: The Yorklyn Railroad Cut

Rock Outcrop Bb25-c: The Yorklyn Railroad Cut

Wissahickon gneisses and amphibolites are exposed in the railroad cut near Yorklyn. Here the rocks are unusual because the layering is accentuated by the presence of fault gouge between the layers. Fault gouge forms as movement along a fault in hard, brittle rocks crushes and grinds the rocks into a powder. Gouge was a term used by miners because they could easily "gouge" it out of the rock. Here the gouge "weathered out" leaving deep indentations that emphasize the layering and the tilt, which is to the southeast at an angle of about 45 degrees.

Outcrop Bc32-a: The Mt. Cuba Picnic Grove

Rock Outcrop Bc32-a: The Mt. Cuba Picnic Grove

The Mt. Cuba Picnic Grove provides an opportunity to look at the gneisses and amphibolites of the Wissahickon Formation. The large boulders of gneiss lying beside the steps are peppered with dark-red garnets and elongated nodules of dull-white sillimanite. These sillimanite nodules (1/4" to 3/4" long) are abundant in the gneisses at Mt. Cuba and are an interesting feature of these highly metamorphosed sedimentary rocks. Alternating layers of gneisses and amphibolites crop out on the east side of the track. The gneisses show some typical upright folds and fractures. Contacts between the layers trend northeast, parallel to the regional trend of the Appalachians.

Outcrop Bc32-b: The Mt. Cuba Railroad Cut

Rock Outcrop Bc32-b: The Mt. Cuba Railroad Cut

The Mt. Cuba railroad cut is narrow and deep, and much of the rock is covered with dirt and soot from the train. The rocks are interlayered gneisses and amphibolites, with gneisses predominating in the south end of the cut and amphibolites in the north end. Folding is well developed, but the angle of the sunlight as it shines on the walls of the cut will determine which of the folds will be the easiest to see. A 4" amphibolite layer outlines the fold in this part of the outcrop. Wonderful examples of the effects of rock type on folding styles can be seen in the cut and in many of the rocks piled north of the cut.

White Clay Creek State Park (NED) Seismic Station

White Clay Creek State Park (NED) Seismic Station. The seismometer, located in White Clay Creek State Park, is positioned on the Wissahickon Formation. The Wissahickon Formation is an extensive sequence of pelitic and psammitic gneisses interlayered with amphibolites. With few exceptions, most of the amphibolite layers are less than 30 feet thick. The rocks have been metamorphosed to upper amphibolite facies and isoclinally folded. The formation is located within the Wilmington North, Kennett Square, West Grove, Newark West, and Newark East U. S. Geological Survey 7.5-minute quadrangles.

Wissahickon Formation

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Interlayered psammitic and pelitic gneiss with amphibolite. Psammitic gneiss is a medium- to fine-grained biotite-plagioclase-quartz gneiss with or without small garnets. Contacts with pelitic gneiss are gradational. Pelitic gneiss is medium- to coarse-grained garnet-sillimanite-biotite-plagioclase-quartz gneiss. Unit has a streaked or flasered appearance owing to the segregation of garnet-sillimanite-biotite stringers that surround lenses of quartz and feldspar. Throughout, layers of fine to medium-grained amphibolite composed of plagioclase and hornblende, several inches to <30 feet thick or as large massive bodies, are in sharp contact with the psammitic and pelitic gneisses. An attempt has been made to show some of the amphibolites mappable at the scale of the map. Granitic pegmatite is ubiquitous and occurs at all scales. Pyroxene bearing quartzite with garnet occurs locally near the contact with the Wilmington Complex. An ultramafic lens composed of cumulus layers of serpentinized peridotite, metapyroxenite, and metagabbro occurs near Hoopes Reservoir. The ultramafic lens may be correlative with the Baltimore Mafic Complex.

RI60 Geochemistry of the Mafic Rocks, Delaware Piedmont and Adjacent Pennsylvania and Maryland: Confirmation of Arc Affinity

RI60 Geochemistry of the Mafic Rocks, Delaware Piedmont and Adjacent Pennsylvania and Maryland: Confirmation of Arc Affinity

Geochemical data from Ordovician and Silurian mafic rocks in the Wilmington Complex in Delaware, the James Run Formation in Cecil County, Maryland, and the Wissahickon Formation in Delaware and Pennsylvania were collected in conjunction with preparation of a new geologic map of the Delaware-Pennsylvania Piedmont. Although concentrations of most elements may have been disrupted by metamorphism, the more stable high field strength elements, including the rare earth elements (REE), are consistent within mapped lithodemic units and are compared to modern basaltic magmas from relatively well known tectonomagmatic environments.

Brandywine Creek State Park (BWD) Seismic Station

Brandywine Creek State Park (BWD) Seismic Station. The seismometer located at Brandywine Creek State Park is positioned on the Wissahickon Formation. The Wissahickon Formation is an extensive sequence of pelitic and psammitic gneisses interlayered with amphibolites. With few exceptions, most of the amphibolite layers are less than 30 feet thick. The rocks have been metamorphosed to upper amphibolite facies and isoclinally folded. The formation is located within the Wilmington North, Kennett Square, West Grove, Newark West, and Newark East quadrangles.

SP4 Generalized Geologic Map of Delaware

SP4 Generalized Geologic Map of Delaware

The Generalized Geologic Map of Delaware is a brief summary for general use indicating the major types and locations of rocks present throughout the State, and their interrelationships. The map is preliminary as it is a first step in a continuing program of detailed geologic mapping. It is based upon many existing sources of data; additional detail may be found in the references listed.

Woodlawn Quarry: A GeoAdventure in the Delaware Piedmont

Woodlawn Quarry: A GeoAdventure in the Delaware Piedmont

A visit to Woodlawn Quarry is suitable for ages 10 to adults and provides an interesting opportunity to observe common mineral specimens, identify the quarry as an early mining site, appreciate the physical work necessary to quarry rock with hand tools, and discuss the economic importance of the minerals found in the quarry. The minerals that can be readily found and identified in the quarry are feldspar, quartz and mica.

OFR14 Sinkholes, Hockessin Area, Delaware

OFR14 Sinkholes, Hockessin Area, Delaware

Sinkholes are depressions in the land surface or holes in the ground caused by subsidence or collapse of surficial material into openings in soluble rock. Sinkholes usually develop in "karst" areas underlain by carbonate rocks. Karst is defined as "terrane with distinctive characteristics of relief and drainage arising primarily from a higher degree of rock solubility in natural waters than is found elsewhere" (Jennings, 1971, p.1). In addition to sinkholes, other features associated with karst are: caves, disappearing streams, and well-developed subsurface drainage systems.

Piedmont Rock Units

The Piedmont occurs in the hilly northernmost part of the state and is composed of crystalline metamorphic and igneous rocks. This chart summarizes the age and distribution of the geologic units that are recognized in the Delaware Piedmont by the Delaware Geological Survey.

What are GeoAdventures?

The Wilmington Western Railroad follows the Red Clay Valley through the Delaware Piedmont cutting through many of the Piedmont rock units.

GeoAdventures are designed to allow the reader to learn about a particular geologic point of interest in Delaware’s Piedmont province and then take a short field trip to that area. Want to know more about the Wilmington blue rock or Brandywine blue granite? Take the Wilmington Blue Rock GeoAdventure and go see just what the blue rock looks like.

Overview of the Piedmont

The Piedmont is defined by hard crystalline rocks north of the fall zone.

The Appalachian Piedmont and Atlantic Coastal Plain are physiographic provinces that are separated by the fall zone. The fall zone (also called the Fall Line) is the contact where the hard crystalline rocks of the Piedmont dip under and disappear beneath the sediments of the Coastal Plain. The landscape and rock types shown in northern Delaware are classical examples of the larger geologic features that dominate the geology of eastern North America.