Gettysburg’s Place in Geologic Time

This is the first of a series of posts that will be published over the course of this summer (mini series!). The goal of these posts are to educate people who visit or live in Gettysburg, Pa about the natural history of the area, why this place looks like it does and eventually tie it in to how the geology affected the battle. I (Jake) am a geologist who now works as the Bicycle Mechanic/Advertising manager for GettysBike Tours. I will try to make these posts as interesting as possible and try not to get to technical! If you have any questions as your read through these posts (as well as others) feel free to leave us a comment and I will get back to you as soon as I can. In this post I hope to give you a basic understanding of the current geologic environment and where we came from (in geo terms).

Gettysburg, Pa is located near the base of the Appalachian mountains. If you were to take a slice straight through PA you would see many ridges (mountains) and valleys. Gettysburg is located in one of those valleys. The valley we are in is called the Gettysburg Basin. –Just a brief note, a valley is generally a basin as it is a low area where fluids and sediment accumulates.– The Gettysburg Basin was formed and filled over millions of years by slow geologic processes during the Mesozoic Era. This era is divided into three periods, most of which you should be familiar with! The Triassic (251-199.6 million years ago), Jurassic (199.6-145.5 million years ago), and Cretaceous (45.5-65.5 million years ago)… the age of dinosaurs! And before you ask yes there are dinosaur fossils but well get to that in a later post! So as time moved on the sediments turned to rock and filled in the basin. Much like the Appalachians used to be many times taller, the Gettysburg Basin would have been many times deeper.


Where did the sediment come from? Well it came from the Appalachians as they eroded over time (mostly). As the range eroded sediments where deposited into sheet like layers (thats a generalization as there are also lenticular beds and other forms but lets not get to carried away for this discussion), these sheets are assigned times which tell us when it was eroded and deposited. The make up of these layers or beds help tell us about Earth’s ancient climate (more on this in another post).

So that is how the basin came to be right? No. Thats how the basin came to be in the early Mesozoic, at this point all that is in the basin is a bunch of sedimentary rocks. On a slight side note (but related) the Gettysburg Basin is part of a larger basin called the Birdsboro Basin, This Basin was formed due to the split up of Pangaea. Today there are a bunch of intrusive igneous rocks. If you have ever been to or seen pictures of the battlefield you would have seen the large boulders scattered throughout (especially in Devils Den). These are intrusive igneous rocks.

To understand where these came from a little back ground in the big picture is helpful. During the Jurassic Pangaea began to split apart. In a simplified version, North America split apart from Africa creating the Atlantic ocean. When continents split apart they become stretched and intern thin over the point where they separate. As they thin, faults form and create an easy path for magma to intrude into the crust. This is one way a volcano can form, though this did not happen here! This process (albeit a very basic explanation of it) is why the boulders that provided great cover for Civil War soldiers are where there at today. Back in the Mesozoic these intrusion would have been buried in sediment, in the time since, the sediment has been eroded into river systems and ultimately out to the ocean (a basin).


This image shows a model of a rift basin the red lines are intrusive magma. This particular image is of the Newark Basin but for our purposes we will consider it the same as the Gettysburg Basin.

These intrusive rocks are called diabase and form dikes and sills. For those that are interested they contain the following minerals calcic plagioclase and clinopyroxene and quartz. There is also trace amounts of magnetite, biotite and olivine. These types of rocks are more resistant to weathering than the sedimentary rocks around them and so over time the sedimentary rocks erode leaving behind the huge boulders.

Many people (especially from the Northern US and Canada in glacier territory) may think that these boulders were placed by glaciers. This is not so since the glaciers never reached this far south. Their weathering was affected by the colder climate during the time of glaciers.


To simplify the above map the green represents the furthest glaciation while the yellow shows the most recent.

Age wise the ground and mountains you see, both here and in the distance, (Appalachians) are very old. Not the oldest in the world nor the oldest in the country but they are old and have been standing there in one form or another for a very long time. The hills you see on the battlefield (for the most part) are predominately made up, in their core, of the intrusive diabase rocks. If these intrusions never happened then the Gettysburg area would have just been a flat plane (mostly). This would have made cemetery hill strategically useless (nor would be a hill) same with Little and Big Round Top.


Stay tuned for more on the geology of the battlefield. The next post will be a Gettysburg Battlefield history post!




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