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Woodlawn Quarry: A GeoAdventure in the Delaware Piedmont

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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 observe 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 part of the First State National Monument, a Federal National Monument within the National Parks System.

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, it is now part of the First State National Monument, a Federal National Monument within the National Parks System and no mineral collecting is allowed.