A Diabase is black to dark gray to dark green rock that is made up
of crystals large enough to see with the unaided eye. It's made of
minerals heavy in iron like magnetite, augite and hornblende and a
lot of others. In the field you probably wouldn't be able to tell
the difference between a gabbro and a diabase. A diabase is a
variety of gabbro so they look much alike. One of the
distinguishing minerals in diabase is LABRADORITE. If a crystal is
large enough you'll see flashes of color when you turn the specimen
correctly; you'll see mostly blues. Lot's of times you'll see
diabase used as architectural stone and for decorations and
gravestones because they are harder and tougher than regular
gabbro.
Pennsylvania's Diabase
A much younger group of igneous rocks, which formed during the
Jurassic Period, less than 200 million years ago, cuts across this
older assortment. They consist of a dark, medium- to fine-grained
intrusive rock, diabase. Diabase is a mafic rock that is believed
to have formed from magma originating in the mantle. It has a
different mineral composition than the older ultramafic rocks that
have altered to serpentinite. The origin of the diabase found in
Pennsylvania is related to the movement of large, continent-sized
plates. The diabase is found in a place that was being rifted, or
pulled apart, when North America and Africa were separating,
forming the Atlantic Ocean. Magma from the mantle entered the space
that separated the two sides, forming igneous rock that, with
sediments, filled the widening gap.
The rock face along the eastern side of the ridge of Governor Dick
and large boulders are formed from Triassic Diabase. The diabase
resulted from an extrusion of molten rock up through the existing
shales and sandstone that make up most of the rock in the area. The
diabase is extremely resistant to erosion. The rock that formed the
Devil's Den, Little and Big Round Top battle sites at Gettysburg
National Military Park is of the same diabase.
Park in the big lot at the new Environmental Education Center
located on Pinch Rd. above the village of Mt. Gretna.
To get to Some of the rock: Pick up a trail at the far corner of
the parking area. Take the trail back towards the Environmental Ed
Center, passing behind it. At the first intersection, continue to
the right and down hill. The trail continues to curve to the right.
Keep your eyes open on the downhill/left side of the trail. You
will see some rock in the woods. These are the tops of the
boulders. This is the Colflesh area. It is the closest and easiest
area to find at Gretna. There are many more areas for the intrepid
adventurer.
Triassic diabase has a granite-like feel and is probably best
described like the dried skin of an elephant. Rounded edges and
sloping top outs are abundant. As the angle increases the holds get
smaller and sharper.
1. At the posted coordinates, note your elevation on your
GPSr.
Email me the elevation. DO NOT post this in your on-line log. Your
post will be deleted.
2. Find some of the diabase boulders, email me the color of this
boulder.
3. Post a photo that shows your caching party, your GPSr, and the
boulder.
