Throughout my many hiking ventures in Northwest Philly's Wissahickon Valley Park, I always seem to admire shiny rocks along the trails and come home shaking sparkling sand from my beach towels. The question has gripped me for too long: what makes the rocks here sparkle? (I’m not the only one who’s been wondering!) So I decided to dig into the mystery.
It turns out a few rocks in the valley could be responsible, according to geological researchers whose work I reviewed. Schist is one common rock in the area, recognizable by distinct layers that can be seen without a microscope. Much of the schist layers are heavy in mica, a type of mineral that can have a silver, sparkling appearance. This is probably what I’m seeing the most along the trails!
But, that's not the only shiny rock around. The mineral kyanite, which typically resembles a large blue or grayish crystal, can also be found in schist and could be adding to the twinkle factor. Additionally, quartzite, a crystalline rock, is common to the valley and is sometimes even interlayered with schist. Even the base rock of the area, called gneiss, has big crystals and wavy banding. No wonder the rocks are shining!
If studying rocks interests you, the Wissahickon is a great place to visit! The area as a great place to observe the remains of tectonic plate activity, weathering, and erosion which have been uniquely preserved for millions of years. Beyond being pretty to look at, they are also some of the oldest rocks in Pennsylvania — dating back to Precambrian and Lower Paleozoic times.
Have you noticed any neat or shiny rocks in the Wissahickon? If so, be sure to snap a pic and send it our way!
