In which the blogger considers movement and being alive while biking in the park, hiking in the Wissahickon Valley, and missing an earthquake.
In these dog days of summer, I retreat into air conditioned isolation, cursing the heat and the Dog Star for bringing it. The other morning, though, I broke free, taking a bike ride for many miles through a local park. In an AC cocoon it’s easy to lose contact with the natural world, but the park was awash in meandering butterflies – mostly Eastern tiger swallowtails and black swallowtails, often settling foolishly on the asphalt in front of me. Birds darted across the path into and out of the woods. At one juncture, two goldfinches exploded into flight from the grass to one side of the path, flashes of brilliant yellow. Two males, I have to assume. Many cardinals swept from bush to bush. Given their dull coats, they all seemed to be females. Perhaps, the males among them were masked by the wear and tear of a long spring and summer. Despite the oppressive heat, there was movement everywhere. I was surrounded by movement. Life continues, and continues with vigor.
Movement is an attribute we ascribe to the living. If it moves, we often say that it’s alive, whether or not the object under observation is animate, sentient, or in any other way really living.
Earlier this year, I hiked through the Wissahickon Valley with my sister and her husband. We had in hand a printout of James Alcock’s The Virtual Geologic Tour of Wissahickon Creek, Philadelphia, Pennsylvania. Alcock, a professor of environmental sciences at Penn State Abington College, studies metamorphic rock, using them as a Rosetta Stone to geological history. No better place for that than the Wissahickon. There, memorialized in the stone, is a geological history of millions of years of powerful movement, metamorphosing sedimentary rock into the schist and quartzite of the Wissahickon Formation. During a history of mountain building episodes and clashing continental masses that reaches back perhaps 500 million years, the rock here was folded, folded again, bent back on itself, twisted, thrust skyward, fractured, . . . . The pictures below do not do justice to the geological wonders that mark every turn on hiking paths in the Wissahickon. These show some of the upwardly thrusted layers, the interleaved layers of schist and granitic material, and the sculptured contortions of the rock.
On the microscale, the scene is just as awesome. Peeking out of the rocks and often weathering out and collecting on the ground, are minerals that were caught up and baked in the schist – garnet, staurolite, and kyanite – among others. The picture below is of a very small portion of a slab of schist embedded in the ground and was shot through a 20X power hand lens. Not very high tech, hence the poor quality of the photo. A garnet glitters to the right in the blurry foreground. The elongated dark blue crystals are kyanite, I believe.
Though the rocks around us in the Wissahickon had been in violent motion, now everything, fixed in the act of moving, seemed so geologically stable, so dead. That’s the way it is on the East Coast of the U.S. The earth doesn’t move . . . . Hmm, actually, it does, and just last week it moved to the tune of a magnitude 3.6 earthquake centered several miles from my home in suburban D.C. My only regret is that I wasn’t home at the time, being sequestered in air conditioning on Long Island. To be honest, this earthquake may not have had a tune at all. The tune of the one major, devastating earthquake I’ve experienced (in Lima, Peru) was perhaps not a tune either, though it was sound – the coming and going roar of a locomotive.
Last week's quake reportedly originated in the Pleasant Grove fault zone, some 4 miles down in billion year old rock. There are competing explanations for the quake. One centers on movement sparked by stresses and strains associated with plate tectonics; the other posits that it’s part of a continuing rebound after the massive weight of mountains and perhaps glaciers have been removed (the former by erosion, the latter by melting).
My favorite statement about this quake came from Scott Southworth, a U.S. Geological Survey scientist, as quoted in a Washington Post article:
Nothing to worry about but nice to know the Earth is alive and kicking.
Other Online Resources on the Geology of the Wissahickon
Friends of the Wissahickon website – Geology
Sarah West’s Gems of the Wissahickon
Colleen Gasiorowski’s Philadelphia’s Wissahickon Valley: Its Geology and Geography, Middle States Geographer, 1997