AN OVERVIEW OF THE TECTONIC HISTORY OF GETTYSBURG PENNSYLVANIA
by Jacob Auer
Jacob Auer majored in Geology at Allegheny College and is the current owner of GettysBike Tours
Editor's Note: Diagrams and pictures can be found at the bottom of the blogpost.
Before we get into the origins of the rocks at Gettysburg let's define some vocabulary that will help with the understanding of the following concepts. The first concept is time. Our perspective of a long time ago is, generally, a few thousand years ago into BC. For this discussion we must go back even further, all the way to the Proterozoic Era. This period began 2.5 billion years ago and ended 540 million years ago. Geologic time usually means talking in the millions of years ago scale. Figure 2 shows the breakdown of geologic time and will be helpful to refer back to throughout this reading. Next we need to define an orogeny. A basic definition of this is a mountain building event caused by the collision of two or more continents.
The geologic history of the Gettysburg area begins in the Proterozoic Era, during the Grenville Orogeny. Nearly all of the crust at that time collided and formed a supercontinent this event is called the Grenville Orogeny. The core of the Appalachian Mountains is a result of the metamorphism, volcanism and sedimentation that occurred as a result of the Grenville Orogeny. The metamorphic rocks at the core of the mountain chain are over a billion years old and were deposited over 100 million years.
During the late Proterozoic (800-600 million years ago) crustal extension created fissures where large volumes of magma flowed. The volcanism lasted tens of millions of years and alternated between intrusive dikes and sills and flood basalts. The rocks produced by this volcanism covered the basement rocks formed during the Grenville Orogeny with metabasalts, greenstones and rhyolitic breccias. The mountain used by the Confederates to shield their advance and retreat is comprised in its core of these Rhyolitic breccias. As extensional tectonics continued the supercontinent broke up into a basin, forming the Iapetus Ocean. The basin was filled with deposits from alluvial fans and turbidity flows. Eventually these deposits formed many of the rock units that make up the present day Appalachians.
Through the Cambrian a marine environment deposited coarse grained sand which became sandstone that through metamorphism formed the Antietam Sandstone. This is representative of a nearshore environment. Since the sandstone is more resistant to weathering it is what caps many of the ridges in the area (figure 3).
To simplify the geology of Gettysburg I am going to ignore the Taconic (440-420 million years ago) and Acadian Orogenies (360 million years ago). A more detailed write up of these will be published on our blog but for the purposes of this pamphlet are not necessary. It is important to know that during these orogenies, particularly the Acadian Orogeny, the Iapetus Ocean was closed as the North American plate collided with the African Plate. The Alleghenian Orogeny (325-265 million years ago) formed the supercontinent Pangea. This was the last major orogeny of the Appalachians.
After this orogeny the mountain elevations are estimated to have been 6100 m (20,000 ft) comparable to the modern day Himalayas. Erosion over the millions of years since has lowered the elevations to 730 m (2,400 ft).
During the late Triassic (230-200 million years ago) Pangea began to split apart into roughly what the continents are today. This break up created the Atlantic Ocean by way of multiple half graben and block-fault basins (Figure 1). Old thrust faults formed during the Paleozoic reactivated but instead became normal faults.
This break up created a basin at the foot of the Appalachians called the Gettysburg-Newark Lowland section (also called the Birdsboro basin). This basin contains two deep basins on either end (Gettysburg and Newark basin) connected by smaller “narrow neck” sub-basins. The Gettysburg basin contains strata dipping 25-30 degrees and is 18 miles wide.
The Gettysburg Basin collected sediments from marine environments which formed the New Oxford Formation, the playal lacustrine sediments of the Gettysburg Formation and the fluvial sandstone of the Heidlersburg Member. The Heidlersburg Member is a group of rock formations that can be grouped together due to similarities in overall environment. In the New Oxford formation vertisols and calcrete with alternating lake deposits suggest a semiarid to more humid climatic fluctuations. As the basin expanded and deepened sediments from the surrounding area filled the basin and formed new rocks layers. Some of these sediments include the red shales and sandstones of the Gettysburg Shale of the Heidlersburg member.
Volcanism formed the igneous dikes and sills of the region which make up the core structures of most of the ridges on the battlefield. The Largest of these intrusions is the Gettysburg sill at 500 m (1,640 ft) thick. The heat from these intrusions caused localized metamorphism creating aphanitic hornfels. The mama filled faults, cracks and bedding planes. More on this in a bit!
So what are bedding planes?
Bedding planes are the surfaces that differentiate sedimentary rock layers. If you drove here on Rt 30 from Pittsburgh you passed many great examples of this. The road cuts through the mountains exposing layers or 'sheets' of rock usually at angles and sometimes folded, although the angling and folding occurred after deposition. The distinct layers are separated by bedding planes. Figure 4 is showing the bedding planes at the railroad cut in Gettysburg. This is not a great example but should give you the idea! Note: It is illegal to go down on the tracks without permission as it is CSX property. If you want to see these rocks in person have a look from the road or by one of the monuments!
These are called red beds and would have overlain much of the battlefield before they were eroded! These sedimentary rocks that are 240 million years or even older!
To recap, dikes and sills are intruded magma which when cooled become Igneous rock. The rock that this is intruded in can be nearly any type but in the case of Gettysburg it is Sedimentary, mostly sandstone and siltstones. The type of igneous rock the magma becomes depends on how it cools and its composition. The slower it cools the larger the crystals, the faster it cools the smaller the
crystals. This is why obsidian, which cools fast, looks like glass. The rocks here cooled faster forming crystal grains that you can see!
The name of the type of igneous rock formed here at Gettysburg is Diabase (Between Gabbro and Basalt). In particular the formations are called the York Haven Diabase and the Rossville Diabase (figure 3) and has been dated to approximately 201 million years old (although the York Haven is slightly older). This means they formed during the Early Jurassic. The surrounding sedimentary rock is from the Upper Triassic. The York Haven Diabase forms the Sills and the Rossville Diabase forms the dikes.
The dikes are relatively fine grained while the sills are coarse grained and contain more feldspars. The dikes cooled faster than the sills. This could have occurred for a variety of reasons including depth, amount of material, and water content to name a few!
The diabase contains minerals of calcium plagioclase, clinopyroxene with quartz, magnetite, biotite and olivine. If you don't know what most of those minerals are that's ok! Google is your friend, there isn't enough room to get into those details here! That being said the plagioclase and pyroxene are visible as black and grey crystals.
Do not break open the rocks as this illegal but if you find one that has broken naturally and recently, these grains will be more obvious! Figure 6 shows Bowen's Reaction Series. To make it simple it shows which minerals form at which temperatures. If you have more questions about this ask Jake! Bowen’s Reaction Series shows the crystallization of silica minerals in a magma through the temperature range. Essentially if a magma cools and maintains a high temperature it may form olivine. If that same magma is allowed to cool further it can ‘react’ with the surrounding magma and form pyroxene. This process continues down. Remember it's the cooling of magma with the parent material that forms the minerals so when you look at the diagram start at the top!
Where are they?
The diabase rocks described here can be found at Devil's Den (figure 5), Little Round Top, Big Round Top and spread throughout the battlefield! Devil's Den is perhaps the best example and most spectacular to look at. Little Round Top and, in the fall and winter, Big Round Top will give you a great view of the entire area from their peaks.
Diabase is more weather resistant than the surrounding sedimentary rocks and so has remained after the sedimentary rocks were reduced back to sediment and transported away. This is why Little Round Top and Big Round Top stand as tall as they do. At their core is more diabase acting as a support system for the sediment, in their case it is a sill. Seminary Ridge is formed from a diabase dike. The individual boulders are remnants of a huge diabase sheet that was intruded into the rock formations called the Gettysburg Sheet.
After the igneous intrusions (200 million years ago) the area under went isostatic uplift. This forced the crust upwards exposing it to more erosion. Since the igneous rocks are harder they were left behind as the sediments were eroded by streams. These harder rocks form the core of hills in the area including Seminary Ridge, Culp’s Hill and Little Round Top. The sediment eroded from this area found its way to the cost and formed what is now the coastal plane.
I hope you enjoyed this information, if you would like more info about the geology or what we offer visit gettysbike.com!
National Park Service US Department of the Interior, 2009, Gettysburg National Military Park & Eisenhower National Historic Site Geologic Resource Inventory Report. https://irma.nps.gov/DataStore/DownloadFile/426477 Accessed April 24, 2018
USGS, 2017, What is Geologic Time?: https://geomaps.wr.usgs.gov/parks/gtime/index.html
Buiter, S., Pfiffner, O., Beaumount, C. 2009, Inversion of extensional sedimentary basins: A numerical evaluation of the localisation of shortening: Earth and Planetary Science Letters.
Geology In, 2014, How Does Bowen’s Reaction Series Relate to the Classification of Igneous Rocks? http://www.geologyin.com/2014/09/how-does-bowens-reaction-series-relate.html