That's Some Great Schist!!! EarthCache
That's Some Great Schist!!!
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These coordinates take you to a plaque located in Wissahickon Park
along Forbidden Drive. The path is tightly packed earth suitable
for biking, walking and strollers.
Wissahickon Schist is all around you in Philadelphia. It is that
rock that looks all sparkly and breaks in oddly straight, flat
pieces. This rock is Wissahickon Schist, a variety of schist named
for the Wissahickon Creek where the stone was first studied. It was
named by Florence Bascom, the first woman geologist. Schist is a
class of metamorphic rocks that has strong, usually thin and often
irregular layering created by planar mineral grains that grew in
the rock in response to high temperature and pressure. Wissahickon
Schist was used as a building material because of its
attractiveness up until the early twentieth century.
For a virtual geologic tour of Wissahickon Creek, please click
here.
To log this cache: I would like to see you standing behind the
plaque. As this is not always easy to do alone, a picture with a
minimum of your GPSr and the plaque is required. Additionally, I
would like a picture of a random piece of Wissahickon Schist that
you can find anywhere on the trail. I've included pictures of what
I expect to see. Finally, I need you to measure the 'trend' of the
rocks behind the sign. These rocks have formed from the
metamorphism (changes) to rocks under great pressure. The mineral
tend to align at 90° to that pressure. So therefore, the rocks
break along those mineral planes. Geologists can use this
information to work out the direction that the pressure was coming
from (up and down, side to side, N-S, E-W etc) by finding the
general direction the rocks are tending, and then the pressure was
90° to that! So, using information, you should be able to email
what directions the pressure was coming from to form these rocks. I
reserve the right to delete logs that do not following the logging
requirements.
Geologists believe that 550 million years ago, the park was covered
by an ocean. Sand and clay settled on the bottom forming thick
layers. As more of these layers accumulated, the bottom layer began
to compact to form enormous rock units of sandstone and shale. When
the massive plates of the earth's surface collided, they produced a
great mountain belt from New England to the southern states. The
rock units were deep underground at this point where the tremendous
heat and pressure transformed them from sedimentary rocks into
metamorphic ones, melding the sandstone and shale into Wissahickon
Schist. Geologists use the set of minerals found in the rocks and
their chemistry to estimate temperatures and pressures that
affected the rock during metamorphism. The minerals found here
imply temperatures higher than 550° C at pressures equivalent to
burial 20 to 25 km beneath the surface of the earth. It also caused
rock strata to be bent, cracked and tilted which is seen today as
steeply angled layers. Millions of years of erosion (mainly due to
the Schuyllkill River) have brought the schist back to the surface.
Human beings have also helped expose it to view by extensive
quarrying.
Wissahickon Schist is characterized by alternating layers of
minerals, mostly mica and quartz. The quartz was derived from the
sandstone while the mica was formed from the ancient shale. The
mica is a shiny, flaky mineral which gives the schist its glassy or
metallic appearance while the quartz is often chalky white or
clear, though it may also occur in smoky blue, gray, or other
shades. The quartz is hard enough to scratch the surface of a coin.
Other minerals found in the Wissahickon Schist include feldspar
(often pink, perhaps glassy, hard like quartz, frequently blockish
in shape), biotite (a black form of mica), and garnet (small
pellets of reddish-brown).
Wissahickon Schist being made up of multiple minerals influences
the rocks as a whole. Because of its molecular structure, mica has
cleavage which is a tendency to fracture along planes. This often
causes the Wissahickon Schist to split into think layers parallel
to the cleavage of the mica. This splitting produces the typical
contours of the rock outcroppings. It also provides seepage outlets
for groundwater. Quartz on the other hand is difficult to break due
to it having no cleavage.
Quartz resisted the heat and pressure of progeny, and by doing so
it created features known by the imported terms bouginage and
augen. A bouginage (from a French word for sausage) occurs when
quartz or feldspar, sandwiched between layers of mica, bulges out
and pinches in like a sausage, or in some areas like a string of
sausage links. A bouginage can be about the size of a football. An
augen is much smaller, typically the size and shape of a human eye.
An augen originates during metamorphism when less resistant
minerals such as mica slide across more resistant quartz or
feldspar, so that the harder mineral begins to roll, creating the
elliptical shape. For examples of augen and bouginage structures,
look at the outcrop on Neil Drive northwest of the Falls Bridge.
Also visible at this site is the type of structure called a
crenulation, a series of small folds in the rock caused by bending
under high temperature and pressure.
Thanks for visiting the first and only EarthCache in
Philadelphia!
If you're interested in Waymarking, this cache is also a waymarker
of the same name:
That's
Some Great Schist!!!. Feel free to log it!
Additional Hints
(No hints available.)