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Exploring the Wilmington Blue Rocks: A GeoAdventure in the Delaware Piedmont

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Blue Rocks at Greenway

The Wilmington blue rock, Delaware's most famous rock, underlies both the city of Wilmington and the rolling upland north and east of the city. It is best exposed along the banks of the Brandywine Creek from south of Rockland to the Market Street Bridge. Along this section the Brandywine has carved a deep gorge in the blue rock. The water fall along this four mile gorge is approximately 120', and in the 17th and 18th centuries provided water power for one of the greatest industrial developments in the American colonies. The field trip stops described below are chosen as good examples of blue rock along the Brandywine Creek, and to illustrate how the geology has influenced the development of this area. It is not necessary to visit every stop to become familiar with the blue rocks, you may choose to visit only a few.

The specific objectives of this adventure are to:

  • Examine the igneous and metamorphic rocks of the Delaware Piedmont that have been called the Wilmington blue rock by quarrymen and the Brandywine Blue Gneiss of the Wilmington Complex by geologists.
  • Investigate the role played by the blue rocks in the ancient geologic history of northern Delaware which involves subduction of tectonic plates, formation of volcanoes, and the progressive collision of the North American, European, and African plates to form a huge mountain range. All of this tectonic activity occurred sometime between 570,000,000 and 250,000,000 years ago. Since that time northern Delaware has remained tectonically quiet as the mountains have slowly eroded their debris of clay, sand, and gravel, onto the continental shelf of the Atlantic Ocean.
  • Recognize how the bedrock and accompanying land forms have influenced land use and industrial development. At its peak in the 18th century, the Brandywine Creek flowing across the blue rocks provided energy for some 130 flour mills, paper mills, and textile mills. Later in the 19th century, in the gorge above Wilmington, the duPonts began the manufacture of gun powder, and from their beginning along the Brandywine, they have grown to be one of the giants of American industry.

GEOLOGIC SETTING

The rocks you will see on this trip are locally called the Wilmington blue rocks or Brandywine Blue Granite. When found in stream beds, yards, or old quarries, the rocks are black or dark gray, however when freshly broken during quarrying the rocks are a bright royal blue. Although weathering changes the color, construction workers have always called this rock the "blue rock". Recognizing the importance of these rocks to the city, Wilmington's original baseball team called themselves the blue rocks, a name that has since been adopted by the city's new baseball team.

Geologists map the blue rock by its geologic name "the Brandywine Blue Gneiss" and assign the rocks to a geologic unit called the Wilmington Complex. The Wilmington Complex forms the bedrock under the much of the city of Wilmington and Brandywine Hundred (Figure 1). The rocks are mostly a mixture of metamorphic gneisses and plutonic igneous rocks. The gneisses, which are the most abundant rock type, are the true "blue rocks". However when you see them today along the Brandywine, they are massive, solid, blue-gray rocks with few visible features to indicate their long history. Since their formation approximately 570,000,000 years ago, these rocks have experienced a long history of burial, high-grade metamorphism, deformation, uplift, and erosion. The metamorphism has totally recrystallized the rock to produce a monotonous body of rock that is wonderfully suited for building houses and fences. It is useless as road ballast as it breaks rock crushers so today the large boulders dug up during construction are usually buried off site.

The mineralogy of the blue rocks is simple, the rocks usually contain only four minerals; quartz, feldspar, pyroxene, and magnetite. Geologists have described this rock as a banded gneiss, even though the light-dark banding is weak and not always present. There are large areas that consist of only light gneiss or dark gneiss. The gneisses weather to form a white rind. It is only then that streaks of minerals up to one inch long can be seen on the white weathered surface. The dark streaks are usually pyroxene or magnetite and the lighter streaks are quartz and feldspar. The banding and the mineral streaks are the only features that are commonly seen in the blue rocks.

The tectonic setting proposed for the origin of the Wilmington Complex is thought to be the deep part of a volcano that developed over an east dipping subduction zone. The subduction and volcanism were early in a series of tectonic events that produced the Appalachian Mountain System. Later, probably between 480,000,000 and 440,000,000 million years ago, the volcanoes collided with the ancient North American continent. Because of this collision, the rocks of the ancient continent, the rocks in the volcanic range, and the rocks lying in the ocean between the continent and the range, were all folded, sheared and buried to depths of 10 to 12 miles where they were metamorphosed by extreme heat and pressure. For many years these buried rocks remained at very high temperatures, somewhere between the temperatures required for high-grade metamorphism and melting (around 1,300°F). Today, after uplift and erosion, the highly metamorphosed rocks are exposed in Delaware in what is recognized by geologists as the metamorphic core of the Appalachian Mountain System.

Coarse-grained igneous rocks are exposed in Bringhurst Woods Park and in the communities of Arden and the Timbers. These rocks probably intruded into the blue rocks and may be younger. They are undeformed and only slightly metamorphosed, thus it is good site to study intrusive igneous rocks (Bringhurst Gabbro GeoAdventure).

Use Figure 1 as a guide to where the 5 stops on this adventure are located.

Stop 1. Brandywine Creek State Park

Park in the lot on the east side of the Brandywine Creek just south of Thompson's Bridge Road. At this stop we will see the contact between the blue rocks of the Wilmington Complex and the metamorphic sedimentary rocks of the Wissahickon Formation. The contact runs northeast at 45 degrees parallel to the regional trend of the Appalachian Mountains, and is exposed along Rocky Run.

There are two options for this stop. Walk (1) follows the southeast side of Rocky Run and will take approximately one and one half hours. Some of the walk includes bushwacking off existing trails so this trip is not suitable for young children. The exposures on Walk (1) are abundant and are good examples of both the Wissahickon and Wilmington Complex rocks. Walk (2) follows the dirt road from the parking lot to the south and will take about one half hour. This is an easy walk and you will be able to see both the metasediments of the Wissahickon and the black boulders of the Wilmington Complex.

Walk (1)

  • Walk south along the Brandywine creek. The hillsides on the east of both the parking lot and the road expose large outcrops of the metamorphosed sediments of the Wissahickon Formation. Many of the outcrops are covered with fungus, making it necessary to look carefully to see the features of these rocks (Area marked A in Figure 2). Cross the bridge over Rocky Run. Take one of the paths that lead northeast parallel to Rocky Run (Figure 2B). A few Wilmington Complex boulders are strewn along the hillside, however approximately one quarter of a mile to the northeast you will encounter a large swale that is literally choked with hundreds of rounded boulders of Wilmington Complex blue rocks (Figure 2C). The boulders are dark, rounded, and show light-dark layering. If you look carefully you may see a few "bright eyes". The bright eyes are grains of black magnetite surrounded by white grains of feldspar and quartz. If you use your imagination, you can see the rocks are looking at you!. Geologists believe this field of boulders is to be a paraglacial feature, formed by freeze and thaw action. The boulders slowly worked their way downslope during the last glacial period, about 10-40 thousand years ago.
  • Cross the boulder field, turn left, and walk toward Rocky Run. Look for a wall of rock bordering the northwest side of Rocky Run (Figure 2D). Wissahickon rocks form the wall and the streambed while the rounded boulders of Wilmington Complex gneisses clog the stream, litter the southeast banks and lie scattered in the flood plain. The layering in the Wissahickon wall rock is irregular and defined by stringers of garnet, biotite and sillimanite in a mass of quartz and feldspar. The garnets are dark red, either oval or round, and may be as large as three quarters of an inch in diameter. The stringers, and any folds that are present, are best seen by standing in the stream and looking upstream. The contact between the Wissahickon and Wilmington rocks is hidden beneath the flood plain.
  • To see the contact, you need to follow the stream to the confluence of Hurricane Run and Rocky Run and stay on the northeast side of Rocky Run. (Figure 2E). The exposed contact is difficult to recognize and probably interesting only to geology students at the high school or college level. It is exposed in a ten foot area along the northeast side of Rocky Run where dark, fine grained Wilmington Complex gneisses are interlayered with light colored Wissahickon gneisses. The Wissahickon rocks appear to have been melted and recrystallized to form granites with thin layers of garnets. The biotite and sillimanite that occur in the Wissahickon gneisses are replaced by tiny garnets. This reaction in which garnet replaces biotite and sillimanite occurs only at very high temperatures. The Wilmington Complex layers vary in thickness between 3 inches and 2 feet, and are dark solid, massive rocks.
  • The nature of this contact is controversial. Geologists are unable to find any substantial evidence in the rocks that will allow them to determine how these two units were placed next to one another. The possibilities are: (1) the original volcanic pile that became the Wilmington Complex rocks was thrust up and over the Wissahickon sediments during subduction of the tectonic plates, (2) the Wilmington Complex slid down from the northeast, maybe from as far northeast as New York City, on a large regional strike slip fault such as the San Andres in California, or (3) that the contact is intrusive and the Wilmington Complex igneous rocks intruded the Wissahickon sediments before the metamorphism.
  • Return to the parking lot.

Walk (2)

  • Walk south along the Brandywine creek. The hillsides on the east of both the parking lot and the road expose large outcrops of Wissahickon rocks. Many of the outcrops are covered with fungus, making it necessary to look carefully to see the individual minerals and the layering (Area marked A in Figure 3). Look for large garnets and curving stringers of biotite and sillimanite.
  • Walk down the road and cross the bridge over Rocky Run. The contact between the Wissahickon and the Wilmington Complex occurs approximately 450 feet south of the bridge. At the contact the rocks in the roadbed change from the light colored, mica-rich rocks of the Wissahickon to dark, rounded boulders of the Wilmington Complex. These Wilmington Complex boulders dot the hillside east of the road. Most boulders are banded and some will contain "bright eyes" The "bright eyes" are grains of magnetite surrounded by light colored quartz and feldspar. If you use your imagination, you will see the rocks winking at you!
  • Return to parking lot.

Stop 2. Rockford Park

This is the most easily accessible stop and will take between fifteen minutes and a half an hour to observe the blue rocks at this location. Follow the main road in Rockford Park to the parking lot at the tower. Park and walk toward the Brandywine Creek. Along the ridge are large outcrops of sharply banded Wilmington Complex gneisses (location of "star" in Figure 4). The banding runs 40 degrees east of north, parallel to the regional strike of the Appalachian Mountain System. The layers are vertical, orientated perpendicular to the land surface.

The bands are 9 to 12 inches thick. During intense metamorphism, around 440,000,000 years ago, these rocks were totally recrystallized and stretched. During stretching, the dark bands were more rigid than the light bands and separated. The light bands were plastic and flowed between the separations. French geologists named this texture boudinage. It is caused by intense squeezing or stretching of the rock while it is warm and plastic.

The light bands are composed of quartz and plagioclase feldspar, with minor amounts (<5%) of orthopyroxene. The dark layers are composed of subequal amounts of plagioclase feldspar and clinopyroxene/hornblende. The composition of the dark layers suggests they were originally basaltic lava flows.

In addition, coarse grained granitic pegmatites intrude the rocks. Pegmatites crystallize from water-rich melts, or magmas, that form late in the metamorphic process. The minerals in the pegmatites are usually very large and easy to identify. In addition rare elements may be concentrated in the water-rich melts and form unusual minerals. Mineral collectors like to search pegmatites for rare minerals. This pegmatite in Rockford Park is dark and weathered, making it is difficult to even recognize even the common minerals in the vein.

Stop 3. Quarries on Brandywine Creek, Alapocas

This quarry has recently been given to the county as part of its park system and can be accessed on the Delaware Greenway (location of "star" in Figure 5). Good exposures of Wilmington Complex gneiss or blue rock are found on the exposed back wall of the quarry. The rock is a monotonous, light-colored gneiss with a few thin dark bands. The dark bands appear to have been deformed by stretching or pulling apart and often occur as pieces about a foot long . Thicker dark bands may persist for the extent of the exposure. The dark bands probably represent original lava flows. This rock looks as if it has been squeezed and stretched. The stretching occurred many years ago when the rocks were hot and plastic. Today these rocks in the quarry are hard and brittle. They will no longer bend or fold, but they will fracture and break during earth movements such as earthquakes or erosional unloading.

Stop 4. Brandywine Park

Large boulders line the banks of the Brandywine as it flows through Brandywine Park. The boulders along the creek are blue rocks, but the banding is replaced by irregular layering and, in some rocks, the mafic bands are replaced by clots or pods of mafic rock (location A, B, C in Figure 6).

This stretch of the Brandywine was the location of many of the mills, thus the bedrock is much disturbed. A large mill race still exists on the southwest side of the creek, however in the 18th century mill races bordered both sides of the stream. The races carried water to turn water wheels and provide energy for the many mills built below the great falls near the Market Street Bridge. Below the Market Street Bridge the Brandywine is navigable, allowing ships to sail up the Christina and lower Brandywine to pick up the flour, cotton, and snuff from the mills that lined the stream.

The rock removed from the mill races was used to build homes for the mill owners and workers. Many of the houses and churches in Brandywine Village that have been built from blue rocks are now beautifully restored.

Stop 5. Swedes Landing

Old Swedes Landing, Fort Christina Park

This stop will take about one half an hour and is an easy interesting walk through the park at Old Swedes Landing to "The Rocks" in the Christina River (Figure 7).

In 1638 the Kalmar Nyckel and the Fogel Grip sailed up the Christina River past the entrance to the Brandywine to "The Rocks" where a large flat slab of blue rock protrudes into the main channel of the river. This rock slab was a convenient place to unload the weary passengers that were aboard the ships. The passengers, mostly Swedes and Finns, stayed and settled on the Christina near this site.

The large flat slab of rock on which the early settlers landed, although reduced to make room for river travel on the Christina, is still a present in Swedes Landing Park. "The Rock" is a slab of Wilmington Complex gneiss or blue rock, and marks the eastern edge of exposure of the Appalachian mountain system where the hard rocks of the Piedmont Province plunge beneath the soft sediments of the Coastal Plain. The boundary between the Piedmont and the Coastal Plain is defined in most places by a well-marked change in topography, usually an abrupt transition from rolling hills to a flat smooth lowland. Geologically it defines the transition from the hard crystalline rocks of the Piedmont to the gently dipping beds of younger clays, sands, and gravels of the Coastal Plain. This boundary is called the Fall Line, and extends along I-95 from Newark, through south Wilmington, toward the Delaware River. It is but a portion of the line or zone that extends unbroken from New York to Georgia. Many of the great cities of the east such as new York, Trenton, Philadelphia, Wilmington, Baltimore, Washington, Richmond Raleigh, and Macon are built on the Fall Line.