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.