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Suburban Philadelphia’s quintessential building material is called “Wissahickon Schist,” and designers from the 18th thru the 20th centuries relied on this durable, easily-mined, and ubiquitous “pelitic schist” to create masterpieces in all styles: Italian Villa, Gothic Revival, Georgian, Colonial Revival, Second Empire… 

Our rampant use of Wissahickon schist is one of the reasons why streets of wildly-varied architecture feel seamlessly tied together, and naturalized with their surroundings. 

Most of us have seen so much Wissahickon schist we’re blind to how lovely it really is: the distinctive flecks of shiny mica and multi-tones of gray, blue, tan, brown and black. But what isn’t so obvious is Wissahickon schist’s unique trick:  the stone is soft enough when quarried to carve with a hatchet or saw, but when exposed to air will quickly oxidize and become as hard as steel.

While homes of all classes were once made from Wissahickon schist, today you pretty much need some serious bank to afford to build with it — hand-tooled stone starts as high as $60 per foot!

A geological perspective, from the first and only EarthCache in Philadelphia

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.

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).

Garnets? It’s true! If you look closely at the Wissahickon schist of old stone walls and footings in our East Falls neighborhood, you’ll often find rust-colored splotches where garnets have oxidized out. In the case of our own Hohenadel House, a quick inspection turns up quite a few small but distinct garnets embedded in the stone foundation:

hohenadel house schist garnet

Indeed, soft Wissahickon “garnet schist” can be very beautiful, and has been highly sought after in architectural construction for decorative adornment alone.  Gem collectors have even found remarkable specimens worth small fortunes on the auction block, including this ghostly silver Almandine garnet found in 1937, and this red-streaked beauty found on the same occasion.

Rock on! Want more info on schist, stony loams and quartzite exposures?  Visit Friends of Wissahickon’s informative page on the geology of the Wissahickon Gorge.