UPDATE 11-23-2022 The foot bridge over the Brandywine is closed and needs extensive repair. Not expected back for 2 years or longer. Now its hike to reach this spot. Option 1 Start at Alapocas Ballfields parking which is free. Option 2 (shorter) Park on Edgewood Rd off Alapocas Dr (waypoint) and use Access Spur Trail (waypoint).
The objective of your quest is the Wilmington blue rock, Delaware's most famous rock. It underlies both the city of Wilmington and parts of Brandywine Hundred. It is best exposed along the banks of the Brandywine Creek from south of Rockland to the Market Street Bridge where the Brandywine has carved a deep gorge in the blue rock.
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".
You will be visiting the site of extensive past quarry activity and also see the now closed Bancroft Mill complex. This area is in Alapocas Run State Park.
To reach this area, requires a substantial hike starting at the large parking lot at Alapocas Ball Fields (see waypoints.
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.
Stop 1 at the posted coordinates is where you can find the signboard explaining this area and the quarries.
Stop 2 is slightly downriver, you will see one of the dams built for waterpower and can learn about the Bancroft Mill. See informstion sign on Bancroft milles. 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
Stop 3 further down river is the very large horseshoe shaped quarry area where there are 100 foot sheer walls.
To log this quest you must answer the following questions
1. What years did the quarry remain in operation? (stop 1)
2. What year was Bancroft Mills founded? (stop 2)
3. Describe the color and texture of the rock at the gorge or quarry (Stop 3)
4. Take a picture of yourself and/or your GPS in front of large horse shoe shaped quarry, (Stop #3) Showing your face is not required.
Technical Information
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. 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".
Since their formation approximately 600 million 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 blue 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 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 metamoric 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.