If you had to pick a soundtrack for the Wissahickon, Queen’s “Under Pressure” definitely would be in the mix. The valley was formed through a dramatic series of geologic events in which tectonic plates collided, buckled, and warped, creating a variety of metamorphic and igneous rock and spectacular formations.

The land itself was once covered in water – and there is ample evidence of its damp past in the signature rocks that we find there today, namely the sparkling and wrinkled Wissahickon schist and the glassy quartzite. The story of that schist – a signature element in Northwest Philadelphia’s architecture as well as its natural environment – is also a microcosm of the larger Wissahickon Valley.

Schist begins as mud and clay sediments that settle out of slow-moving water. When that sediment builds up and compacts over time, the mud turns into sedimentary shale. 

When the tectonic plates of the earth’s crust slowly converged hundreds of millions of years ago, slamming the North American plate into the African, they generated inordinate amounts of heat and pressure. As this pressure grew, our shale metamorphosed into slate. 

That tectonic convergence continued to dial up both heat and pressure, forcing the molecules in the slate to rearrange, and change into phyllite, which looks similar to schist in its sheen – but without the large crystals of mica.

A familiar mineral mix

With continued heat and pressure, the crystals of mica grew, and the phyllite gradually changed to the wrinkled, sparkling schist so familiar to us now, a mineral mix of soft, shiny mica, whose silvery flakes are like geologic glitter, along with the more resilient quartz and feldspar. 

The geologic story of the Wissahickon doesn’t stop there. There is evidence that some areas of our valley are even more “well-baked” than others. Just as stressful times can challenge humans to produce spectacular results, temperatures reaching 1,000 degrees Fahrenheit create “Goldilocks conditions” in schist, and some of the original clay particles arrange into almandine garnets. 

And the Wissahickon is littered with them. 

Although not gem-quality (sorry, rock hounds!), almandine garnets are valuable in helping to tell the story of this region, namely its muddy beginning and its extreme metamorphism. Because they are more durable than the flimsy mica that often encases them, the garnets eventually escape their rock capsules and fall out – becoming an exciting discovery for anyone who goes looking. 

Some of that garnetiferous mica schist kept transforming as continents merged many millennia ago. We know this because we also find a high-grade metamorphic rock called gneiss (pronounced “nice”) in some outcrops in the Wissahickon. Characterized by alternating dark and light stripes (like a “nice” zebra), gneiss forms when schist is heated and pressed, causing the various minerals to break down and rearrange, settling into layers based on density as it cools. 

And if there’s even more heat and pressure? The metamorphic gneiss becomes molten, and you get igneous rock such as granite. 

In the Wissahickon, most of the rock is metamorphic. However, there were some intrusions called plutons (imagine large bubbles) of granite that seeped up into the rock as it buckled and warped. In the 19th century, many of these plutons were quarried for building stone by folks such as Joseph Middleton, who owned a granite quarry between Bell’s Mill Road and Northwestern Avenue and became president of the Wissahickon Turnpike Company. 

In many of the outcroppings you find throughout the park, you’ll see curves and bends in the solid rock. These are souvenirs of the mountain-building that created this region. Just as a car collision will bend and fold solid materials, these dramatic events provided opportunities for molten liquids to seep into cracks, where they eventually solidified.

The harder rocks

If you head north from Valley Green Avenue on the east side of the creek, you’ll come to a magnificent outcrop of folded rock with thick bands of light-colored minerals – mainly mica, feldspar, and quartz – among the darker layers. These are layers of granite that originated as silica-rich magma that flowed between the layers and was folded along with the metamorphic rock. The magma cooled slowly and crystallized. 

The other main metamorphic rock in this region is quartzite. Along the banks of the Wissahickon Creek, you will find weathered samples of many local rocks, along with others washed in from further upstream. Because of the hardness of its main mineral (quartz), quartzite is one of the “blockier” specimens you’ll encounter at the water’s edge. 

Quartzite is metamorphic, like schist, and it has a similar origin story.

While small sediments like clay and mud can settle to the bottom in slow-moving water, they are swept along by faster-moving water. Only larger sediments like sand and pebbles are heavy enough to settle out of fast-moving water. When layers of sand become compressed over time, they form into sedimentary sandstone. And when tectonic plates collide and turn up the heat, sandstone metamorphoses into quartzite. Look carefully at a fresh edge of quartzite and sometimes you can see the fused grains of sand.

Along the Orange Trail by the Mt Airy Trailhead entrance, you can see an outcrop with alternating layers of schist and quartzite. Millions of years ago, water ebbed and flowed over this surface, alternating layers of sediment as the water speed changed. The tectonic collision preserved this story in metamorphic rock.

So while the trails of the Wissahickon can seem peaceful and calm, their rocky, slanted outcrops hold clues to a more tumultuous past, from tectonic collisions and extreme pressure to bending and breaking and cooling. 

And slowly, very slowly, the rocks continue their cycle as they weather and erode, their sediment forming layers, adding pressure, and setting the stage for another chapter of transformation in years to come.