This rock formation, called Devil’s Den, was the site of fierce fighting during the Battle of Gettysburg in the Civil War.
In a treeless plain on the outskirts of Gettysburg in southern Pennsylvania lies a jumble of dark and rounded boulders.
This “bouldering town” consists of diabase rock, a very hard igneous rock that landed here 200 million years ago and was formed as hot magma that seeps from underground volcanoes to the surface. For the past few centuries it has been known as Devil’s Den.
On a hot summer day in 1863, desperate for cover, Confederate soldiers drove Union forces off the rocks, then dug into the cracks and fired at their rivals, who were hiding behind another diabase in Little Round Top showed up.
Other geological forces had long ago helped direct Union and Confederate armies onto this battlefield.
Confederate troops had used the threatening barrier of the Blue Ridge Mountains to covertly move north. A windy gap enabled them to move a large force by wagon through South Mountain and on to Gettysburg, where a surprising encounter awaited them.
Neither side wanted to fight there because the Gettysburg Plain had diabase rocks just below the surface and soldiers could not dig for protection. This meant that the increased outcrops became turning points in the battle.
The size of a car, this sandstone cliff is roughly 350 million years old and will eventually fall over the ridge it is perched on.
The formation of Pennsylvania land features and bedrock has been ongoing for millions of years. Along the way there were dinosaurs, volcanic eruptions, glaciers, erosion and sedimentation, and the crash of tectonic plates that formed mountains and oceans.
These forces have shaped many facets of the state’s landscape, history, and communities, from battlefields and industrial activities based on natural resources to the known fertility of the state’s farmlands.
Geological processes have also provided a multitude of outdoor pleasures: waterfalls, canyons, glacial lakes, river gaps, salt springs from trapped ancient seas, boulder fields, ice mines, “jingling” rocks, and more.
As you explore the state’s geological landmarks, you may appreciate the drumlins, rock towns, and moraines, and you may learn that Pennsylvania was once an ancient seabed – and was south of the equator.
“I just hope people get a little understanding of what’s under their feet,” said Stuart Geese, geologist with the Pennsylvania Geological Survey. “Rocks tell a story. It’s really amazing how much has happened, how beautiful it is and what story can be put together from studying stones. It is wonderful.”
This finger-like sandstone tower offers a sweeping view of the geography of the Pennsylvania Ridge and Valley.
The Geological Survey has compiled guides for 122 locations that can be accessed via an online map at gis.dcnr.state.pa.us/pageode. Warning: Navigating the website may take some time. From the Layers menu, choose to make the trace of geology visible. Click the map icons for individual sites. A brief summary is displayed in the side panel. Click on the Trail of Geology site number found there for more detailed information.
Here is a brief selection of Pennsylvania’s geological attractions to explore.
- Chickies Skirt: This quartzite tower on the Susquehanna River in Lancaster County was likely formed as beach sand deposited there 1-2 million years ago.
- Pole Steeple, Michaux State Forest: A short, steep hike in Cumberland County leads to a dramatic 80-foot ledge that is a holdover from an ancient collision between what is now North America and Africa.
One of the most scenic overlooks in Pennsylvania, Pole Steeple is a quartzite upswing that was caused by immense forces about 300 million years ago.
- Pennsylvania Grand Canyon: Here Pine Creek used to flow north, but the current was reversed as lakes formed by glaciers flowed over the lowest point in the drainage division and eroded.
- Susquehanna columns of water: Fans of Penn State football matches can get a good look at these five dramatic gaps in the mountains from PA Route 322 and US Routes 11 and 15. The exact origins of the loopholes are still being discussed. The most popular theory is that over time the river meandered through weaker ridges.
This boulder field is called Devils Race Course because you can hear a stream gurgling down below.
- Devils Race Course, State Wildland 211: Located north of Harrisburg, this bouldering field was formed when glaciers caused freeze-thaw cycles that slowly moved exposed rocks from downhill. You can hear a small, invisible stream gurgling under the rocks.
The 25-foot-high sandstone block, which overturned in a landslide 8,000 to 10,000 years ago, is embedded along the Susquehanna River.
- Standing stone: Viewed from US Route 6, this 25-foot-high block of sandstone along the Susquehanna River fell there in a landslide 8,000 to 10,000 years ago.
- Cornwall mines and iron furnace:Iron oxides were deposited at this location east of Harrisburg when molten magma cooled 200 million years ago. The iron ore kilns, which operated from 1742 to 1883 and at one point produced cannons for the continental navy. The mine is now filled with clear water.
