Monday, January 20, 2014

At the Edge

Kali and I are having our 25-year-old couches reupholstered and have been working with a fabric and upholstery store located near the mouth of "my" creek.  On Saturday, we went to pick up the finished upholstery, and took a walk along 'my" creek while we were there.

The creek flows south out of my preserve, enters a county park, then crosses the line into the city of Philadelphia where it flows within municipal parkland all the way to its mouth at the Delaware River.  About two miles upstream of its mouth, the creek falls off the hard edge of North America and flows onto sediments deposited on the shallow continental shelf - the Coastal Plain.  The transition from the hard, ancient Piedmont rock to the sandy Coastal Plain - the fall line - is dramatic.
The fall line
The creek rushes over resistant bedrock and creates a series of rapids at the fall line.  Early European colonists took full advantage of the transition by building a dam on top of the rapids and harnessing the water power for a mill - the most southerly (downstream) of the 28 mills that at one time operated along my creek.  In the image above, the rocky ruins of the massive dam are still in place on the opposite side of the creek, and some of the ruins of the mill are visible as a stone wall in the background.

The Piedmont bedrock at the fall line displays graphic evidence of the repeated stresses experienced by the rock at the edge of the continent.  North America and Africa have collided with one another on at least two occasions, and the bedrock, composed  mostly of dull, gray metamorphic gneiss, is twisted and bent like taffy.  In addition, other types of rock have gotten caught-up in the collisions and been incorporated into the cooled "taffy" like the quartzite in the image above, and the granite in the image below.


A stone studded with mica
Silvery flakes of mica are abundant in some of the rocks.  In the sunlight, the rock above glistened hypnotically.  If I'd been much younger and in an acquisitive (rather than de-acquisitive stage in my life), I would have brought the rock back with me.  The image hardly does it justice.
Fish ladder
A few hundred feet upstream of the fall line, state agencies have constructed a fish ladder in the creek.  My creek once was spawning ground for anadromous shad that returned from the ocean to spawn, just like salmon.  With the construction of colonial dams for water power, the shad were excluded from their historic spawning streams and their populations plummeted.  There's an effort to remove dams and restore shad runs in some of their historic streams, including my creek.  This fish ladder was created to allow the shad to swim over a sanitary sewer line partially buried in the streambed; the sewer pipes formed enough of a barrier that the fish could not ascend beyond them, so our Fish and Boat Commission built this series of riffles and pools.  In addition, they released many thousand hatchery-raised fingerling Hickory Shad into the creek in the hopes that some will return to spawn.
The pools created by this fish ladder have become very popular swimming holes with members of the Hispanic community in the neighborhood.  On warm summer weekends, extended families bring barbeque picnics to the edge of the creek to enjoy the water.  Although swimming is prohibited, and the creek is far too "impaired" for safe contact, it doesn't stop the kids from cooling off.  Fortunately, if the shad begin to use the creek for spawning again, they would return in March and not have to contend with crowds of kids.

Requisite sycamore-against-blue sky image
Just a few hundred feet downstream of the fall line, the creek is spanned by the oldest extant bridge in the United States: the King's Highway Bridge, built in 1797.  The King's Highway, constructed during the English colonial period, connected Philadelphia and New York City, and ran approximately along the fall line.  The highway replaced a Native American pathway that traced the same route.  The bridge is a National Engineering Landmark.
The fall line's a pretty "happening" place!

6 comments:

packrat said...

Another fascinating, informative blog post, Scott. Interesting and excellent photos, too. Thanks.

robin andrea said...

It is awesome, in the truest sense of the word, to see a water way that has so much history. From cataclysmic smash-ups with Africa to the earliest extant bridge to fish ladders for shad-- it tells quite a story.

Scott said...

Thank you, Packrat. The image of the King's Road Bridge is not very good, but the sun was almost directly behind the bridge, so it was hard to get a good shot. In fact, the bridge is positioned in a way that I have never been able to get a good image.

Scott said...

I'm glad you enjoyed the post, Robin Andrea. One of the other impressive things about the Piedmont near the fall line (which I did not include in the post) is the fact that the Piedmont rocks are among the most ancient rocks on the earth. They're at least a billion years old. I don't know about other people, but I'm impressed by that fact. Often, when I'm presenting a program and mention the extreme age of the rocks, I usually get blank stares. Maybe people can't conceive of ages like that, or maybe they just don't care--"They're just rocks."

Mark P said...

You've got some cool stuff up there.

Maybe the concept of a billion of anything is too hard to comprehend.

Scott said...

I think you hit the nail on the head about the incomprehensibility of a billion years, Mark.