Posted By : admin
Did Geology Influence the Battle of Gettysburg?

Obviously, the answer is …Yes!  Otherwise, this would be the end of this article.

With regard to precipitous heights, if you are beforehand with your adversary, you should occupy the raised and sunny spots, and there wait for him to come up.

The Art of War, by Sun Tzù


First Defender Dale Kratzer has taught a course at Alvernia University for over 30 years on environmental issues.  It is a course for non-science majors, and the college in which he teaches is geared to adult students.  Generally, these are students who are back after being out of high school for several years, and, science is ‘not their thing.’  So, Dale tries to inject various items to make the course more interesting for them.  This is one such item.

Now, geology has had an influence on any number of battles, but this little article looks only at Gettysburg with a focus on July 3rd.


To understand how geology influenced the battle, a very brief introduction to some of the geologic forces at play may be helpful.  So, to start we need to go back a mere 200 million years or so to the time of the supercontinent known as ‘Pangaea,’ or ‘All Lands.’  At that time all the continents as we know them today were ‘fused’ together in one huge land mass named Pangaea. 


Eventually, tectonic forces came into play which caused the continents to be separated from Pangaea on their own individual ‘tectonic plates.’   “Tectonic” is from the Greek language for carpenter or builder; for our purpose, though, it refers to changes in the Earth’s crust, the forces that produce those changes and the nature of those forces.  For this discussion, the two plates of interest to us with respect to the Battle of Gettysburg are the North American and African Plates.  

(Dale has also found any number of comics that add to the course the way he teaches it, ergo:)

Now, as the North American and African Plates pulled & slid apart a rift valley known as the Triassic Rift Basin was created.  Over time, the basin has filled with eroded materiel known as sediment, and today the original floor is several hundred feet below the surface.  Perhaps a rift basin better known to all of us is Death Valley in California.

But, we also know the Triassic Rift Basin but by different names in different locations.  In South Carolina it is the Low Country, in Virginia it is the Piedmont.  Here in the Keystone State, it is the Great Valley.  The red line on the geologic map below very generally traces the Basin in southeast Pennsylvania through Adams and Berks Counties.  And, although the term is used primarily by geologists, at Gettysburg it is known as the “Gettysburg Basin.”

Over geologic time, sandstones, siltstones and shales were deposited in the Gettysburg Basin.  Today that material collectively is referred to as the “Gettysburg Formation” and is the dominant geologic unit in the battlefield.  Subsequently, the basin was broken by hills and ridges that were formed as a result of further geologic activity when molten magma from below the surface was thrust (intruded) into the Gettysburg Formation.  That molten magma subsequently cooled into an igneous rock material known as Diabase.  Diabase is extremely dense, hard rock and very resistant to weathering (think granite).  The two key features of this diabase of interest to us and the battle are a dense 2000-foot thick, broad, almost horizontal slab of diabase rock now known as the Gettysburg Sill (which we know from the battle’s standpoint collectively as Cemetery Ridge and Hill, the Round Tops, and Culp’s Hill), and, a vertical, sharply delimitated, 50-foot diabase “dike” which we know as Seminary Ridge.

 With that as a little geologic background we come to July 1863

We are all familiar with the ”Fishhook” showing the basic disposition of each army at Gettysburg.  Now, let’s take a look from a slightly different perspective.  The area within that blue line is the very dense diabase ‘plateau’ of the Gettysburg Sill.  The red line to the left is the very dense diabase dike. 

So, NOT DID, but HOW DID Geology Influence the Battle of Gettysburg?

As noted earlier, we’re looking at this primarily from the standpoint of July 3rd, but you can relate this to other actions as well. 

First, Union troops positioned themselves along the ridges and hills of the Gettysburg Sill (plateau) which provided excellent vantage points.  However, the thin soil on many of the sides and tops of hills also made it almost impossible for Union soldiers to entrench themselves.  The resistant diabase bedrock was so close to the surface on these hills that the troops were unable to “dig in.”  They had to rely on existing stone walls, improvised barricades, scattered boulders and outcrops of rock for protection.  As a result, there were much higher Union casualties than if a comparable battle had been fought on more ‘defender-friendly’ ground.  Geologists and historians who have looked at the issue estimate Union casualties were 25-30% higher than if the Army of the Potomac had been able to ‘dig in.’

Next, Confederate troops moved across the cleared valleys and lowlands in an effort to drive the Union troops off Cemetery Ridge, Culp’s Hill, Little Round Top, and Big Round Top.  Diabase is a hard, homogenous rock, only slowly weathering, forming a shallow, rocky soil.  It’s not very useful to grow crops, and was not cleared for farming thus making the terrain even more difficult to cross for an attacker.

The sediments of the Gettysburg Basin and other material, known as hornfels, surrounding the Gettysburg Sill form only an apparent flat, as the sediments eroded into the Basin and the slightly harder hornfels form a gently rising slope towards the diabase “plateau and other diabase outcrops.”  The result is a wide-open field without shelter and an uphill approach.  The slope slowed down the advancing Confederates leading to increased casualties.

Next, consider artillery placement.  Advantage: Union.  Actually, two advantages.  The diabase dike of Seminary Ridge caused the Confederate artillery to overshoot the Union front line with many of the shells landing well to the rear.  Then during Pickett’s Charge the Confederate infantry was an easy target for the Union artillery which was firing from a raised position with increased range and exposing the  Confederates to artillery fire for a longer time.

The sill also contains a large amount of feldspar which is prone to weathering and which is the source of much of our topsoil.  The forces of weathering are evident in the large boulders of Devil’s Den and on the sides of the Round Tops formed from diabase outcrops.  While these large diabase boulders could provide shelter, they also tend to break up a line of soldiers making a coordinated attack difficult.

And, a last comment.  Geology had an influence on the entire Gettysburg campaign.  To wit, Massanutten Mountain and the Appalachians in general provided cover for Lee’s army as it moved north.


Gettysburg National Military Park, Geologic Formations Brochure

Scientific American, Geology and Generals: How Geology influenced the Battle of Gettysburg, July 3, 2014

The Geologic Society of America, Geology of the Gettysburg Battlefield: How Mesozoic Events and Processes Impacted American History

(For more on the impact of geology on the Battle of Gettysburg, consider Troy Harman’s All Roads Led to Gettysburg: A New Look at the Civil War’s Pivotal Campaign)


Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.