It would be nice to age without changes occurring in our
appearance. Unfortunately, human flesh deteriorates over time, and
we witness this deterioration as alterations in our appearance.
Changes in appearance can be seen in non-living things as
well as living things. The earth has changed in its appearance over
the passage of time. Like the flesh of the human body, the rocks
that compose the earth are worn down and deteriorate with age. Many
factors contribute to the breakdown of the rocks of the earth. The
process of breaking down the rocks that compose the earth is called
weathering. Weathering can be chemical or physical.Physical
weathering changes the size of a rock but does not
alter the composition of the rock. Temperature changes can
cause physical weathering to occur. Rocks expand in the summer and
contract in the winter. If you are unclear on the terms expansion
and contraction, think about how tight your rings feel in the
summer and how loose they feel in the winter. In other words, the
fingers swell (expand) in hot weather and shrink (contract) in cold
weather. This alteration of expanding and contracting in rocks
weakens the rock and can cause it to crack and break into pieces.
Roots from shrubs and trees can also grow between the openings in
rocks, causing them to break apart. This also represents physical
weathering.
Chemical weathering may change not only the size but also
the composition of a rock. In this type of weathering, a
new material is formed. Oxidation and carbonization are two
forms of chemical weathering. Oxidation is the process in which
combining oxygen combines with other materials to form new
substances. Some rocks can appear to have a black, brown, or orange
coloration because of oxidation. Iron oxide (rust) develops on
these rocks when the iron mineral in the rock is exposed to the
atmosphere. In effect, the rock has rusted. Carbonization is
another agent of chemical weathering. Carbonization forms limestone
caves when carbonic acid (formed from the union of rainwater and
carbon dioxide in the atmosphere) reacts with limestone rock to
dissolve it slowly. Over thousands of years a cave is formed. As
time goes by, the cave will increase in size.
The Hammer Creek Formation is the predominate geology covering
all of the Triassic Lowland Section with reddish-brown,
coarse-grained sandstone having interbeds of red shale and
quartz-pebble conglomerate. Moderately resistant to weathering, it
varies depending on the predominant rock. Conglomerates weather
least rapidly of all rocks.
Here in the Furnace Hills, this ridgetop outcropping of
quartz-conglomerate has weathered into Eagle Rock. Animal
formations typically abound in Pennsylvania caves, but on the
surface they are rarer. To be certain, Pennsylvania has no shortage
of rock features named after animals, but they are more likely to
be named for the creature that may have INHABITED them rather than
for the animals they happen to LOOK like.
Eagle Rock is located just slightly off of the Horseshoe Trail.
It requires an uphill hike of approximately 1 mile with about a
600' gain in elevation. The trail is rocky and might be difficult
when wet.
A good place to park (if space is available) is at the trail
marker at N40°14.427 W76°20.177 (there is also a State Game Lands
parking area nearby).
Eagle Rock is located on the top of a ridge north of Poplar
Grove, near the Lebanon-Lancaster County line. The Horseshoe Trail
traverses this ridge and bisects the Camp Mack Boy Scout Camp. This
used to be a completely forested ridge top and the view is only now
possible by the cutting of a considerable number of trees directly
below the ridge. The view is north into Lebanon County and the
Lebanon Valley. Blue Mountain is the ridgeline on the horizon.
Outcrops of quartz conglomerate of the Hammer Creek Formation
(Triassic age); one outcrop has weathered unevenly and now
resembles the shape of an eagle. It is said the Indians used this
spot for a lookout.
To qualify as a "find":
1) Upload your/your team photo with GPSr at Eagle Rock.
2) Email (do not post here) which form of "weathering" you
believe caused the shaping of Eagle Rock.
3) Email (do not post here) your reasons to support the answer
you have given to #2.