Woodlawn Quarry GeoAdventure
Woodlawn
Quarry: A GeoAdventure in the Delaware Piedmont
INTRODUCTION
A
visit to Woodlawn Quarry is suitable for ages 10 to adults and provides
an interesting opportunity to collect common mineral specimens, identify
the quarry as an early mining site, appreciate the physical work necessary
to quarry rock with hand tools, and discuss the economic importance
of the minerals found in the quarry. The minerals that can be readily
found and identified in the quarry are feldspar, quartz and mica.
This
area was bought in 1910 by William Bancroft as a wild flower preserve.
It is now managed by the Woodlawn Trustees and is open to the public,
however permission to visit the site must be obtained from the Trustees
office.
Feldspar
was actively quarried at this site from 1850 to 1910. There were many
feldspar quarries or spar pits as they were commonly called scattered
throughout the Delaware Piedmont in the early eighteen hundreds. The
feldspar recovered from this spar pit was transported by horse and wagon
to a factory in Philadelphia where it was used for making porcelain
products such as dishes, figurines, false teeth, or sinks. The quarry
eventually closed because machinery made other sites more accessible.
GEOLOGIC
SETTING
The
rock quarried is an intrusive igneous rock called a granite. Intrusive
rocks do not flow or explode from a volcano onto the earth's surface,
but solidify deep within the earth. Molten rock called magma flows slowly
through cracks or other zones of weakness in the local rock and cooled
slowly to solidify into a rock made up of large mineral grains. The
intrusive rock quarried here at Woodlawn names a graphic granite because
the feldspar grains contain inclusions of quartz in geometric shapes
that look like the cuneiform writing of the ancient Arabs. The graphic
granite also contains white mica (Muscovite) and the accessory minerals
garnet and beryl.
The
graphic granite cooled and crystallized slowly within preexisting rock,
called the country rock. The so-called country rock surrounding the
graphic granite is part of the Wissahickon Formation, a formation made
up of highly metamorphosed and intensely deformed rocks that formed
in the core of the ancient Appalachian Mountains. The magma from which
the granite crystallized probably formed during the metamorphism. This
is a common occurrence in metamorphic terrains where the coarse grained
granites are called pegmatites.
MINERAL
IDENTIFICATION
The
minerals found in this quarry can be distinguished by their physical
properties, color, cleavage or fracture, and luster. Cleavage is the
tendency of some minerals to break along definite surfaces that are
parallel to possible crystal faces, and provides a means of identifying
these minerals. Minerals without cleavage will break by fracturing or
breaking in all directions. Not all minerals show good cleavage, most
show fracture.
FELDSPAR
occurs as two varieties, one is pink and one is white. All the feldspar
grains a re opaque, that is light does not shine through the mineral.
The feldspars break with good cleavage in two directions. The pink
feldspar has better cleavage than the white and often breaks into
small perfect rhombohedrons. The fresh cleavage surfaces have a pearly
luster. The pink feldspar is a variety called microcline, and the
white feldspar is plagioclase. Both feldspars form similar crystals,
but have different elements in their crystal lattices. Plagioclase
grains display surface striations due to exsolution during cooling.
QUARTZ
grains are transparent to translucent, that means that light will
pass through the grains. They occur here as crystalline masses that
fracture like glass. The masses show a transition from clear white
quartz to smoky quartz.
Quartz
is the most common mineral in surface rocks. It is the principal constituent
in many igneous sedimentary and metamorphic rocks and forms the sand
on most of our beaches. It has many uses such as a gemstone, as an
electronic component, as the principal component of glass.
MICA
is easily recognized because it has perfect one directional cleavage
and separates into thin elastic sheets. A cluster of sheets if referred
to as a book and appears block and opaque. The sheets are clean and
transparent, but may contain hexagonal-shaped inclusions (reticulated
inclusions) of a black iron mineral. Separating the books into thin
sheets illustrated the prominent basal cleavage. This colorless variety
of mica is called Muscovite.
The sheets obtained from large books were use to make heat proof windows
for old stoves and ranges. Because of their electrical resistance,
the iron-free micas are widely used in many kinds of electrical equipment.
The isinglass, popular years ago as shatterproof windows in automobiles
was made using a sheet of mica and clear glass.
GARNET
occurs here as tiny dark red crystals with 12 sides, called a dodecahedron.
The crystals are rare and small and it is necessary to look carefully
to find crystals. The garnets are hard, have a glassy luster and no
distinct cleavage. When broken they look like dark red glass.
BERYL
or aquamarine as it is commonly called, is pale blue-green. It has
no cleavage and occurs here as irregular masses in the graphic granite.
Beryllium is a rare element, and most granitic pegmatites do not contain
beryl, however this occurrence is part of a group of beryl-bearing
granitic rocks that have been identified in southern Chester and Delaware
counties in Pennsylvania and northern New Castle county in Delaware.
Both garnet and aquamarine are semiprecious stones.
This
map shows the location of Woodlawn Quarry. As previously stated, the quarry
is on property managed by the Woodlawn Trustees. Permission to visit the
site must be obtained from the Trustees office. You may call the Trustees
office for more information at (302) 655-6209.