The Baltimore area has gneissic rock that originally was part of an offshore volcanic arc called the Chopawamsic Terrane. This stretch runs all the way to Wilmington, DE where it is called the Wilmington Complex. However, around Baltimore it is referred to as the James Run Formation. Ancient tectonic convergence and an eastward-dipping subduction zone on the ocean floor generated igneous activity that formed the chain of volcanoes 488-470 million years ago, and these rocks are the metamorphic remnants. Subduction zones are sites of convective downwelling of Earth's lithosphere (the crust plus the top non-convecting portion of the upper mantle). Subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate. The descending slab—the subducting plate—is over-ridden by the leading edge of the other plate. The slab sinks at an angle of approximately twenty-five to forty-five degrees to Earth's surface. This sinking is driven by the temperature difference between the subducting oceanic lithosphere and the surrounding mantle asthenosphere, as the colder oceanic lithosphere is, on average, more dense. Info from Wikipedia
The Jones Falls Trail is an asphalt sidewalk that leads from Druid Hill and Penn Station. It runs along the Jones Falls as it descends through the gneisses of the James Run Formation. There are a few places where you can see the Jones Falls Gneiss.
How to get there:
There are two ways you can do this Earthcache. The one that will give you the most for your smilie is to park at the given parking coordinate in Druid Hill Park (open sunrise to sunset). From there, walk the road to the given trailhead. Follow the paved trail down to Falls Road. When you reach Falls Road, you need to cross the street to stay on the Jones Falls Trail. If you follow the Reference Point, you can view the lovely Round Falls (really a dam, not falls, but still nice to look at). There is some gneiss here to look at. Note that this OPTIONAL part of the Earthcache is NOT wheelchair or stroller accessible. I would call it a T:2.5.
Continue down the Jones Falls Trail until you are under the 29th Street Bridge. On the eastern side of the roadway, you can see several pieces of the Jones Fall Gniess. I believe the entire path to the Earthcache is doable in a wheelchair. Obviously going back up the hill will be more of an effort, but it is paved all the way.
If you are unable to do the walk, the Gneiss can also be seen from your geomobile as you pass by on Falls Road. Make sure you are on Falls Road, and not on 29th Street, as the coordinates work for both, but the elevation is not the same!
Send the answers to the following questions to show that you were here and learned a few things (all questions are for the gneiss under the bridge, not the optional side trip):
- The gneiss shows many layers/bands. Do the layers seem to go vertically, horizontally or something else? Why do you think that is?
- Does the gneiss appear to be rough or smooth?
- Do the stones use to make up the 29th Street Bridge (the wall behind the gneiss) appear to be gneiss? What appears the same or different?
Point of clarification. The first two questions refer to the gneiss stones in/on the ground. The third question then compares the gneiss on the ground with visible stones used to make up the bridge.
Information for this cache came from "Roadside Geology of Maryland, Delaware, and Washington, D.C." by John Means, mostly pages 130 and 213. Thanks to Firennice, a.k.a. BlueRajah, a.k.a. GeoawareUSA2 for recommending this series during an interview on a geocaching related podcast. And thanks to GeoawareUSA7 for patiently walking me through my first Earthcache.