Once scores of mills lined the banks of the Wissahickon Creek using and reusing the Creek's water to power industries which sawed wood, made paper, ground grain, and manufactured felt. Today nearly all of these buildings are gone, having been destroyed or converted to other uses. WVWA's Evans-Mumbower Mill still stands, preserved and open to the public, offering a glimpse into the past. A visit provides an understanding the history of an old Wissahickon mill and how important such buildings were to the Wissahickon Valley economy.
The Wissahickon Gorge belongs to the Piedmont Province of Pennsylvania, a strip of sloping land rising from the Atlantic coastal plain on the eastern side of the Appalachian Mountains. The Piedmont zone consists of Precambrian and lower Paleozoic rocks, the oldest rocks in Pennsylvania.
The Wissahickon Creek and its tributaries pass through several geological regions including the Newark Basin of Triassic sandstone and shale, the limestone and dolomite of the Chester Valley and the Wissahickon Formation where its waters merge with the Schuylkill and Delaware Rivers. In its last seven miles, the Wissahickon stream drops over 100 feet in altitude.
Origin of Wissahickon Rock
Once buried 25 kilometers below the surface, Wissahickon rocks provide evidence of the powerful tectonic forces that can close oceans and build mountain ranges. At one time, geologists hypothesized that Wissahickon rocks were part of ancestral North America, but a major fault, the Huntingdon Valley shear zone, abruptly separates the Wissahickon Formation from very different rock types to the north.
The concept of terranes offers a plausible explanation of the origins of Wissahickon rocks. For example, they may have been part of an eroded mountain range far older than the Appalachians that rifted from another continental mass and traveled with the oceanic crust, eventually adhering to our continent. During 250 million years of activity along a converging plate boundary, numerous small microcontinents, including the ancestral Wissahickon, torn from their earlier locations, became slowly welded to the North American coastline as the massive Appalachian Range formed.
Types of Rocks in the Wissahickon
The predominant bedrock underlying Philadelphia is Wissahickon Schist. First studied and described along the Wissahickon Creek, schist extends in a broad band across southeastern Pennsylvania from Trenton into Delaware and Maryland. In addition to schist, most exposures show layers of quartzite. Wissahickon schist and quartzite are metamorphic rocks formed from sedimentary deposits of mud and sand that at one time were washed off very ancient continents into a shallow sea. These sedimentary deposits were compressed into shale and sandstone. During the long periods of mountain building, the shale and sandstone were slowly transformed into the schist and quartzite we find today. In some places, the compression and heat were extreme enough to fuse the schist with emerging igneous rocks into hard-banded gneiss.
Layers of pegmatite are also common in Wissahickon exposures. These are igneous flows that were pushed through deeply buried schist and quartzite during the periods of mountain building. Visitors may also come upon old granite quarries--the remains of granite plutons that moved up into overlying rock layers.
Within the schist can be found a variety of mineral crystals, muscovite and biotite micas, garnet staurolite, tourmaline, kyanite, and in some regions, sillimanite. These embedded crystals reveal the conditions of combined temperature and pressure existing in the rocks as they underwent metamorphosis. In a few locations close to Devil's Pool and along Bell's Mill Road, talc schist containing the soft mineral talc that can be scratched with a fingernail is found. This material has risen from very deep within the earth's crust.
Variety of Rock Formations
One of the most fascinating aspects of Wissahickon geology is the wide variety in rock form and substance that can be seen within a short distance. Some rocks have been folded as though they were made of putty while others have been cracked into huge blocks during long periods of compression. A few feet away from rocks that can be scratched with a fingernail can be found rocky ledges so hard they can barely be marked with a steel nail. Weathering may leave one exposure smooth and rounded while nearby the rocks may be towering and flat, craggy or rough. One can't help but wonder about the tumultuous forces within the earth's crust producing this great variety.
In order to claim credit for this Earthcache you are required to message the CO with the answers to the following questions:
1) For what purpose was the rock from the quarry used.
2) What type of rock do you think is found in the creek bed and why do you suppose this was a good location for a dam.
3) In which direction does the rock ledge run in the creek bed? Parallel or Perpendicular.
4) At the Quarry Waypoint, with your back to the creek and looking at the rock ledge, it appears fractured and sits at a downward angle from left to right. Estimate the incline angle in degrees from the horizontal. Hint: there is an app for that.
5) Describe what colors and texture are present in the rock at this location.