West Mt. Airy in the Stone Age


WEST MT. AIRY IN THE STONE AGE

by Burt Froom

West Mt. Airy:  Yesterday and Today

January, 2013 (Article 4)

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With this article, we will begin to look at the world of the Wissahickon Creek Before Europeans arriving. Today we will consider the stone called Wissahickon schist. Then, we will look at the creeks, then the forests, and then the wildlife of  this lost world. I thank Sarah West and Dr. James Alcock, retired professor of geosciences at Penn State Abington and a West Mt. Airy resident,who helped me to see the world of millions of years ago.

Today is a good day to celebrate Wissahickon schist (pronounced shist)! This sparkling, gray stone appears often in West Mt. Airy and in adjacent communities. We see it in houses and walls, looking substantial and mellow. Our schist is a medium grade metamorphosed stone derived from shale that contains small amounts of many minerals including light mica (muscovite), biotite, quartz, and feldspar. Wissahickon schist was known to early European colonists who called it “glimmer stone.” The word schist comes from the Greek word shistos, meaning to split, from its layered structure.

Schist is 500 million years or more old. Dr. Alcock says there are dark gneiss basement rocks in the Wissahickon Creek near to Northwestern Avenue that are more than 1 billion years old. (http://www.personal.psu.edu/faculty/j/e/jea4/VWiss/BellsMill.html) Wissahickon schist is a medium hard rock with glints of brown, green, red and blue. It has been widely used since earliest colonial times because it was plentiful, strong, attractive, and its layered structure made it useful material for building construction. Though Philadelphia is largely a city of brick, West Mt. Airy displays many stone houses built prior to World War I.

The geology of the Wissahickon valley is a fascinating story. The Wissahickon Creek ravine is largely made of schist, together with gneiss, quartzite, and pegmatite (a kind of granite), in its towering walls, up to 380 feet high, and in the creek bed, which drops 100 feet during its last eight miles. On a visit to the Wissahickon, we gaze upon a great wound in the earth’s skin where we see the results of erosion by the creek. Dr. Alcock estimates that the Wissahickon Creek has been flowing in its present bed and direction (east and southeast) for just a few million years. There are few places in our restless world, and so close to a large city, where we can see the mighty geological forces that created the Grand Canyon of Arizona – which recent findings show may have begun 70 million or six million years ago. The Wissahickon and its predecessors, is thought by geologists to be older.

 

Outcrop of gray mica-rich Wissahickon schist and white granite near Valley Green Inn. The bands of granite follow the contorted layering of the schist.
The younger white granite appears to have intruded into the older folded and deformed schist.
(James Alcock, http://www.personal.psu.edu/faculty/j/e/jea4/VWiss/Wisstopo.html)

This geological formation was named “Wissahickon schist” by the first woman geologist in America, Florence Bascom (1862–1945), who studied it first in the Wissahickon valley. Bascum founded the geology department at Bryn Mawr College, where she taught for over 30 years.

The convoluted schist that we see in the Wissahickon underlies West Mt. Airy and much of Philadelphia. Actually, this Wissahickon schist formation extends in a broad band across southeastern Pennsylvania from Trenton and into Delaware and Maryland.

Well, where did this great belt of schist come from and how did it get formed? Though I am not a geologist, my investigation has led me to this dynamic picture of the formation of schist in our Wissahickon Creek valley:

Let us take a fast but long trip to look at the world that produced our Wissahickon schist:

Before Philadelphia became such a great city,
Before William Penn and his Quakers settled here and before the Swedes came around here,
Before the Lenni Lenape aboriginal people lived here,
Before modern human beings (the Cro-Magnon) emerged 40,000 years ago,
Before the age of the great mammals like the woolly mammoths that became extinct during the last ice age,
Before the great meteor collided with our planet 65 million years ago off the coast of Yucatan, Mexico, and exterminated all the dinosaurs (except the birds),
Before the dinosaurs ruled the earth for 185 million years,
Before fish learned to walk on land,
and when the trilobites (related to today’s insects, spiders and crustaceans) ruled the oceans,
500 million or more years ago, Wissahickon schist began to be formed!

The geography of the earth around 500 million years ago was very different from today.  According to the theory of continental plate tectonics, which science first understood in the 1960s, there were then two super continents, Laurasia to the north and Gondwana, to the south, moving toward each other, riding on the earth’s mantle. Between the two continents lay the Iapetus Ocean. Recent scientific thought theorizes that in that ocean there were micro-continents which were the source of our Wissahickon schist, and that some of these small continents became attached to Laurasia and the east coast of America.

Conjectural Pangaea, “Entire Earth”, circa 300-200 million years ago, with modern continents outlined. From Wikipedia

 

Geologists tell us that for countless millennia, rivers draining the east coast of America and these micro-continents eroded high mountain ranges, carrying fine clay and sand to thick layers in the ocean. After millions of years, the Iapetus Ocean began to narrow, when all of the world’s land masses slowly collided against each other to create Pangaea (from the Greek, entire earth or all lands). The southern mega- continent, Gondwana with Africa, collided against the northern mega-continent, Laurasia with eastern United States, 450-300 million years ago.

Many scientists believe that during this slow crash, the western edge of the tectonic plate carrying Gondwana (ancestral Africa), dove under the eastern edge of Laurasia (ancestral North America), a process geologists call subduction. Later, the two continental crusts were too buoyant to be subducted  and the crusts crumpled together to create a new range of high mountains. This smash up, beginning about 470 million years ago, started the ancestral Appalachian Mountains, stretching from Alabama to Maine, eventually reaching almost as high as the modern Himalayas!

The layers of stone that became the Wissahickon valley were subjected to tremendous heat and were buried up to 15 miles below the surface. They were compressed, intermingled, folded, and cemented together, about 440 million years ago. Eventually, Pangaea split apart, by about 200 million years ago, following plate tectonic dynamics, to situate earth’s continents in their present positions.

Let us now return from 500 million years ago to the present. The Wissahickon valley is a beautiful natural laboratory for exploring geologic processes. We remember that geologists classify all rocks into three main groups: igneous (originating in molten volcanic rock which flows between other rock types), sedimentary (originating from eroded older rock material that is ground up by wind and water, and is deposited in the ocean), and metamorphic (rock changed chemically and structurally by heat and pressure). During compression and heat, the minerals within metamorphosed stone regroup and re-crystallize to form layers and “gems,” mineral crystals, such as garnet, staurolite and tourmaline.

Schist was originally eroded clay that was buried and compressed to form a sedimentary rock, shale. Deeper burial heated and compressed the rock even more turning the shale to slate. then to  schist, and later, perhaps, into gneiss. Schist is a medium hard rock that is used as a decorative stone, rather than a weight-bearing stone. Gneiss is often harder than schist and is schist fused through extreme compression and heat with emerging igneous rocks and with a distinct banded structure.

Quartzite is a hard metamorphic rock that was originally sandstone and was converted into quartzite through heat and pressure. It may be white, gray or pink. Pegmatite is an igneous, granite rock that contains large feldspar, mica, and quartz crystals. Granite is massive (lacking internal structures), hard and tough, and therefore is widely used in construction. One wonders at the tumultuous forces within the earth’s crust that produced this great variety of rocks!

Schist, gneiss and granite were excavated locally, during the 19th century and earlier, for building stone from Wissahickon area quarries, near Rittenhouse Town, Germantown, lower Chestnut Hill and along Forbidden Drive near Bell’s Mill Road. The bustling McKinney Quarry excavated stone on six acres and some 70 feet into its hillside, at today’s Wissahickon Avenue-Rittenhouse Street intersection. The quarry operated for 70 years until 1910, and employed 75 workers to operate engines and derricks.  Besides schist, it produced granite curbing and Belgian block to pave city streets, and miles of roads.

For further reading, I recommend Sarah West’s, Rediscovering the Wissahickon, and Gems of the Wissahickon, which discuss the origins of the valley and contain self-guided tours. Sarah, long a West Mt. Airy resident, taught earth sciences at Germantown Friends School and is active in the Friends of the Wissahickon organization.