Overview: From July 1st until the
3rd in 1863, two great armies clashed in the struggle which was to
forge a new nation, a truly United States of America. The scene of
one of these battles is your destination, seeking to understand how
the geologic setting was in fact the underlying factor which lead
to major military decisions and outcomes. The formations and
structure of the rocks are simplistic when compared to the rest of
the Piedmont province and yet set the stage for historic events and
outcomes that were to alter the course of history for a young
nation coming to grips with itself.
Battle: The
Battle of Gettysburg was essentially an effort by the Confederates
to drive the Union army from the outcrop of the Gettysburg sill
(York Haven Diabase) south of the town of Gettysburg. This outcrop
is shaped like a fishhook extending northward about 3 miles from
Round Top through Little Round Top and Cemetary Ridge to Cemetary
Hill, the east and south to the barb of the fishhook, Culps Hill.
Devils Den was finally taken by units of Gen. Longstreet's Corps on
the second day, July 2, 1863, and served as a position form which
sharpshooters were able to assault the Union forces on the Little
Round Top. Meade was forced to withdraw units on Culp's Hill to the
north to strengthen the left flank. Gen. Edward Johnson of Ewell's
Corps was within striking distance of Meade's reserve artillery,
and Meade's line of retreat along the Baltimore Pike, but failed to
move forward and thus let victory slip away. Gen. Longstreet
withdrew at the end of the day having lost some 5,000 men.
Geologic
Setting: The Gettysburg Battle Park is set in and
around the small township of Gettysburg in Adams County
Pennsylvania, in the southwest corner of the Newark-Gettysburg
basin, one of several Triassic basins extending from the
Connecticut Valley in the north to the Wadesboro area in southern
North Carolina. The Gettysburg portion of the basin is a
half-graben, down faulted on the western edge in contact with South
Mountain, the northern most part of the Blue Ridge Province. The
floor of the basin is under-laid with gently dipping redbeds of
Triassic age sandstones, and shales that have been intruded by
Triassic/Jurassic age diabase sills and dikes. It is the more
resistant diabase intrusions which form the topographic features
that standout in higher relief throughout the battlefield, and thus
played a major role in the evolution of the three day battle.
Simply put, it was these high ground features for which the two
armies contested.
Geology: Around
you are massive boulders of the diabase sill, York Haven
composition, illustrating the effects of differential weathering.
The joint patterns play a major role in this process, allowing the
decomposition of the diabase to increase the spacing between
boulders. Scaly-like patterns may be observed on some of the rock
surfaces indicating micro-fractures in the diabase. Also, on the
surfaces of some of the boulders you may see raised ridges that may
be mineralogically different from the main body of the diabase,
containing concentrations of ferro-silicates (hornblende and
biotite). Perhaps during later stages of intrusion, iron bearing
solutions entered the micro-fractures which were forming during the
cooling process.
Although the park does not contain volcanoes, canyons, or other
such grandiose geologic formations, its topography and geologic
features are equally important in the influence that they had on
the historic Civil War battle. Approximately 180 million years ago
during the late Triassic Period, the Gettysburg Formation
comprising sandstones, siltstones, and shales was deposited in a
large carved out basin in the Gettysburg area. These lowlands were
broken by hills and ridges that were formed as a result of geologic
activity when a dense 2000-foot thick slab of igneous (molten) rock
called the Gettysburg Sill and also two 50-foot dikes were thrust
into the Gettysburg Formation. One of the dikes underlies Seminary
Ridge in a north to south orientation while the other parallels the
ridge to the west. Sills are responsible for the topographically
high areas of the Round Tops, Culp’s Hill, and Cemetery Ridge and
Hill.
To Claim this cache:
At the given coordinates, you will first need to look up and tell
me:
1. How the large boulder
above you appears, what does it look like and how do you
think it got this way?
Turn around towards the parking area, tell me about the large
boulder on the ground:
2. What kind
of shape does it have?
3. Take a
picture of yourself in your favorite place at Devil’s Den but
please do not include photos of the two boulders listed above.
There are several carvings in the rocks, one
is the basis for a
Virtual (Unsettled Spirit -
GCKA5M) at this location. More information
is available Here.