Wilimington Blue Rocks Geo Adventure
Exploring the Wilmington Complex (The Wilmington Blue Rocks): A GeoAdventure in the Delaware Piedmont
INTRODUCTION
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 (B,
Figure 2). 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 (C,
Figure 2). 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 (D,
Figure 2). 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. (E,
Figure 2). 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
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 1838
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.