Devil’s Den Part I: Geologic Origin

    This is a two part post where we will discuss Devil’s Den. In part I we will talk about the geology of Devil’s Den and the second part will talk about what happened there on July 2nd 1863. Devil’s den is one of the more popular sites in Gettysburg, especially for children. It is a large field of boulders at the base of Little and Big Round Tops. These boulders continue west under the ridge along Sickles Ave.


Image from (Chris Heisey Photographer 2009)

   To understand where these boulders came from you have to understand a little bit about the distant pass. During the Triassic (251-199 million years ago) and Jurassic ( 198-145 million years ago), plus a little bit into the Cretaceous (144-65 million years ago) the Gettysburg area was part of a very long basin stretching from Virginia to Connecticut called the Birsdboro basin. This basin was formed by the pulling apart of the supercontinent Pangea (see image below). This particular split, formed the Atlantic Ocean and Africa. As the continent pulled apart the crust thinned and sunk (both by faulting and simply the weight) forming a low relief area of which the streams coming off the mountains and plateaus could drain and deposit sediment into. Now, this is a very simplified version of the whole process, but it works for our purposes. During the Triassic and early Jurassic, sediment eroded from the Appalachians as was deposited in the basin filling it up and eventually solidifying into rock. On a brief aside: The present day basin contains up to 20,000 feet of rock which at one point was essentially sand eroded from the Appalachians. This may have you asking , How tall must the Appalachians have been to supply this much sediment. The simplified answer is they were as tall or in some cases taller than the modern day Himalayas. In fact at the time of the Late Triassic the elevation of the mountains were probably still 1.2-2.4 miles tall. Most of the sedimentation was finished by the end of the Triassic however some amount of sedimentation will continues for many millions of years.

Rift basins_enoRiverPub_25.png

Graphic showing the Triassic rift basin and the split of Pangea. The brown blob below the “a”in America is the Birdsboro Basin.

  During the beginning of the  Jurassic, magma found its way up through the faults and fissures in the rock that made up the basin. This Magma would never reach the surface in volcanic fashion but would solidify within the basin rock forming the York Haven Diabase. The York Haven Diabase has been dated to 200 Million years old.


   Fast forward a few million years once the magmatism had subdued and the basin stopped growing. Rivers began to erode sedimentary rock layers exposing the more resistant York Haven Diabase. Diabase is a dark colored igneous rock. It is similar to Gabbro and Basalt chemically but is texturally different. The rate and method of cooling controls the texture. This type of rock is found most often in shallow dikes and sills.


These are examples of Gabbro and Basalt.


   So we now know where the rocks at Devil’s den came from, now we need to understand why they look the way they do. The process of physically and sometimes chemically modifying a rock by natural processes is called weathering. The rocks at Devil’s Den underwent spheroidal weathering. This is a Chemical weathering process which affects jointed rocks and results in a  spheroidal shape. The rocks also underwent exfoliation. This is a chemical weathering process which causes the rocks “flake” off pieces, kind of like peeling the shell off a hard boiled egg. The reason this happens is due to the way the constituents of the rock bond together. The driving factor of the exfoliation is a process called frost wedging. Frost wedging is exactly what it sounds like. Water finds it’s way into cracks, it then freezes and expands forcing the rock apart at the crack. This happens due to constant freeze/thaw cycles brought on by colder climates during times of glaciation.


Image from

   The York Haven Diabase which makes up Devil’s Den also makes up the core of most of the high ground in Gettysburg. These areas include Big and Little rounds Tops Seminary and Warfield Ridges. Devil’s Den is the only area where the diabase is exposed.

   It is entirely possible that many 1000’s of years in the future erosion could expose seminary ridge and the round tops so much that they resemble Devil’s den. If you would like more detail on any of these processes just comment below on this Facebook or WordPress post and i’ll be glad to explain!


Stay tuned for Part II where we will discuss the events of the July 2nd 1863!


Thanks for reading!



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