15 July 2018

Chickie’s Rock, Pennsylvania

Posted by Callan Bentley

I’ve mentioned that there is an anticline at Chickie’s Rock, where Chickie’s Ridge is denied projecting any further south by the Susquehanna River. It’s a site I visited last month on a field trip associated with the National Association of Geoscience Teachers’ Eastern Section annual conference. Today, let’s take a closer look at the rocks exposed at the rest of the site.

Let’s begin our explorations with primary sedimentary features, since they are oldest…

The site is composed principally of quartz arenite (quartz sandstone to quartzite, depending on metamorphic grade), which is a tan white when fresh, but obscured in most places by weathering, lichen, railroad soot, rockclimber chalk, and graffiti.

Here’s a clean, fresh bit, just to show that this really is a light-colored rock, not a jet black one:

Also, there are Skolithos tubular trace fossils:

Here’s a set of 3D ripple marks:

Next let’s consider the lovely tectonic cleavage imparted on these rocks by Appalachian mountain-building:

Sorry there’s no sense of scale in that image – it was too high up to safely reach. I’d say it’s about 2 meters wide. Here it is annotated:

It really seems to refract when crossing more pelitic (mud-rich) layers:

Another example, with my field notebook for scale:


My favorite example of the phenomenon was this dismembered syncline hinge with a cleavage fan:

NAGT Eastern Section past president Dave Ludwikoski for scale. Annotated:

We went to another site, too, to look at a coarser facies of the Chickies Formation. The site was Sam Lewis State Park, and it showed off outcrops of the Hellam Conglomerate member of the Chickies Formation. In places, it looks like this:

And in other places it shows a more pronounced tectonic fabric, with flush grain boundaries parallel to a weak foliation, implying some pressure solution has taken place:

Unfortunately, some of the outcrops are covered with graffiti:

The best outcrop there was this one, with a clear bedding surface (sand below, gravel above) and a tectonic cleavage cutting through both layers:


And what’s up with this round patch of clean (relatively lichen free) rock, showing significantly less relief than the surrounding pebbles, which weather out in 3D? I have no idea.

All told, I found Chickie’s Rock a satisfying field trip stop: it had some primary features and some secondary structure, and both were fun to explore.