WHERE IS THE WISSAHICKON CREEK?
by Burt Froom
West Mt. Airy: Yesterday and Today
February, 2013 (Article 5)
I hope that in these column articles you are enjoying our exploration of Wissahickon schist stone and the story of the Wissahickon Creek. There is grandeur and there are surprises in this narrative. Again, I thank Dr. James Alcock, retired professor of geosciences at Penn State, Abington, for helping us understand these geological processes.
Our world in West Mt. Airy is shaped by the three streams that flow in our midst. We have, of course, the Wissahickon Creek, running deeply in its gorge of hard schist, universally loved and enjoyed, visited by 750,000 people each year. It is an iconic and isolated expanse of nature in an urban setting, where driving through it is forbidden, where we must visit on foot (or on two wheels or on four hoofs), where we step into restored wilderness
Then, we have the Monoshone Creek which drains most of West Mt. Airy, which is largely invisible because it flows in deeply buried stormwater pipes under winding Lincoln Drive. The Monoshone Creek carved out a twisting valley with its steep hillsides and charming, picturesque homes, built mostly after 1900. Now, instead of water running through an ancient vale, we have a steady torrent of racing automobiles that endanger the lives of pedestrians trying to cross Lincoln Drive!
And we have the Cresheim Creek which has carved out its own valley as a tributary of the Wissahickon, seen most readily from the McCallum Street Bridge. In the wider world of urban Philadelphia, our two large rivers, the Schuylkill and the Delaware, with much parkland along their banks, are a constant and usually calm presence in our city. We are a blessed city, to have such majestic rivers and pretty creeks!
The Wissahickon Creek runs in its deep valley 380 feet below the Crefeld School in Chestnut Hill, and it covers 1,800 acres in Fairmount Park. The Lenni-Lenape name for it meant, ”catfish creek” or “stream of yellow fish,” It is beautiful in all its seasons: the deep snow of some winters, the lovely leaf buds and flowers opening in spring, the cool creek flowing in summer heat and shadows, and, of course, in the leaves of many colors in fall. Walkers, bicyclers, joggers, and swimming dogs all enjoy this seemingly tranquil world that is the gift of water flowing for millennia. In all, the Wissahickon drains 64 square miles of land and three-quarters of that watershed lies outside the city. One-hundred sixty-thousand people live there. But sometimes the beauty and tranquility of the Wissahickon turn angry and dangerous.
Early in the evening of September 29, 2004, a Tuesday, a big unexpected rainstorm struck Northwest Philadelphia and much of the region. The storm was the angry descendent of Hurricane Jeanne and it delivered as much as ten inches of rain here in just two hours. One survivor of this deluge was a young woman in her 20s, whom I will call “Carol,” who told her story in the Inquirer. The Schuylkill Expressway was jammed with stalled traffic, so Carol took a chance and exited her car onto Lincoln Drive – though she had never driven there before! She was in contact with her father by cell phone. Suddenly, in the darkness, her car was hit by “a wall of water” from the overflowing Wissahickon Creek (caused by too much erosion and paved-over land upstream). Soon, she felt her car floating in water maybe five feet deep. Other cars were floating about, bumping into each other. Carol said she thought, “So this is the way it ends…” She saw people standing on their car hoods and roofs. She felt panicky, but managed to climb out. Then police officers appeared, paddling rubber boats, and they rescued Carol and many other motorist in the rushing waters of the Wissahickon. Carol was reunited with her father later. Yes, the Wissahickon is picturesque, it but can be perilous, too.
Map of the Wissahickon Creek watershed showing the upper valley (tan), middle valley (light green), and lower valley (dark green), and Huntingdon Valley-Cream Valley Fault line (red). To magnify, click on the image and a large pdf will load.
Let us imagine taking a walk down the Wissahickon Creek, just to get acquainted with “our” creek. (For map and information, go to Wissahickon Valley Watershed Association www.WVWa.org), This stream arises in the parking lot of Montgomery Mall. It will travel 21 miles to its mouth at East Falls on the Schuylkill River, 13 miles of which are in Montgomery County, and eight miles in Fairmount Park in the city of Philadelphia. The creek has three distinct regions: The upper valley begins around North Wales, where Merck, the pharmaceuticals giant, is located on its banks. Soils here are made of slates and sandstones and are silty loam – some of the most productive agricultural land in Pennsylvania.
In the middle valley (roughly Springfield and Whitemarsh townships), the creek meanders through limestone and dolomite rocks that produce soils that are slightly alkaline and very fertile. There are active farms here dating from colonial times.
In the lower valley, our “Wissahickon,” the creek becomes more energetic as it erodes its way deeply into the steep descent (a 100 foot drop in elevation from Northwestern Avenue to the Schuylkill), soils weathered from the schists and gneisses of the lower valley are acidic, clayey and relatively infertile. (For map and information, go to Friends of the Wissahickon www.fow.org.) The Wissahickon is surprisingly short but it has three identities. All these environments must have been heavily wooded and the gorge inaccessible to the first Europeans who arrived before 1683. Later, by the 19th century, more than 50 mills were built along the entire Wissahickon Creek and its tributaries.
In just the place where the meandering creek plunges into its gorge at Northwestern Avenue, walking south on Forbidden Drive, we encounter an unexpected phenomenon that we want to understand. It is here, walking south, just before a small hill appears to our view along Forbidden Drive that there is a little waterfall in the creek which is the first outcropping of schist we see walking south. To the north of that falls are outcrops of one billion year old gneiss rock. Though it is not a visible geological feature, this is the location of the Huntingdon Valley-Cream Valley Fault that stretches from Trenton to Chester County. We can place one foot in ancestral North America, built here of gneiss stone, and the other foot in the world of Wissahickon schist. Because of the different stones and their different ages, it is thought that the schist is from a different source, perhaps a micro-continent in the ancient Iapetus Ocean, left here by the collision of ancestral Africa and North America (when Pangaea was formed), maybe 300 million years ago! There are two ancient worlds for us to see in the Wissahickon, just a few miles from home!
Dr. Alcock says the Huntingdon Valley-Cream Valley Fault is perhaps 200 million years old and it is still active. Every 20 or 30 years or so, there is an earthquake that registers a two or three on the Richter scale – not strong enough to do damage. The south side of the fault is lifting up and the north side is moving downward. That movement over any millions of years uplifted Chestnut Hill. As you walk or drive up Germantown Avenue (from Northwestern Avenue) to the Top of the Hill, you are climbing the ancient fault escarpment, now considerably worn down. Walking inside the awesome gorge, we see the slope, created by the fault that the rushing Wissahickon Creek has carved out as its home.
How old is the Wissahickon? Again, Dr. Alcock estimates that the creek in its present bed and direction is only “a few million” years old. That age needs to be explained. Geologists believe that prior to that time, the Wissahickon, flowing south, met the barrier of the fault scarp of the Huntingdon Valley-Cream Valley Fault near what is now the stables on Northwestern Avenue and the creek was forced to turn right in several turns. Almost certainly, the proto-Wissahickon, blocked by the Chestnut Hill barrier, emptied at that prior time into the Schuylkill River in Andorra. Next month, I will explain how the Wissahickon came to follow its current course.
And in the March issue of this Yesterday and Today column, we will look at Creeky West Mt. Airy! If you have any comments or questions about these explorations into our community’s history, please contact me at Froom1@verizon.net. I look forward to hearing from you!