"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
The battleground of Gettysburg was shaped by ancient tectonic movements, sediments transported by rivers and deposited in lakes and finally volcanic eruptions almost 200 million years ago. During the break-up of the supercontinent of Pangaea rift-basins opened, in which rivers deposited sand and mud coming from the eroding Appalachians mountains in the west. This 610m (2.000 ft) succession of sandstone and shale is defined as Gettysburg Formation, with the weathering resistant sandy beds known as Heidlersburg Member.
In the thinning and weakened crust igneous rocks intruded, forming vertical, sharply delimitated dikes and broad, almost horizontal sills. The volcanic intrusions, only slightly differing in petrological composition and age, are classified as Rossville Diabase and York Haven Diabase (diabase is the plutonic equivalent of basalt). Where the molten magma intruded into the Gettysburg Formation, a contact zone of metamorphosed Hornfels formed. Diabase is the hardest rock of this series and around Gettysburg forms the most prominent heights, like Seminary Ridge, Cemetery Ridge, Round Top, Little Round Top and Culps Hill. Especially these heights will be decisive in the outcome of the Gettysburg battle in July 1863.
Fig.1. In this topographic map of Gettysburg from 1863 the heights formed by the more weathering-resistant rocks, and the troop positions of the two armies, are well recognizable (image in public domain).
On July 1 Union troops encountered scouting Confederate forces west of Gettysburg. The loamy ground here is characterized by gently rolling hills and north-south trending ridges, formed by the tilted and slightly more weathering resistant sandstone beds of the Heidlersburg Member. The ridges and - at the time - an unfinished railroad cut provided shelter for the Confederate soldiers and the Union troops retired towards Gettysburg, planning to take the 18m (60 ft) high diabase hills located south-east of the town.
Diabase is a hard, homogenous rock, only slowly weathering it breaks apart in great blocks. Forming a shallow, rocky soil, it's not very useful to grow crops and was therefore not cleared for farming, making the terrain even more difficult to cross for an attacker. Large diabase boulders could provide shelter, however also tend to break up a line of soldiers, making a coordinated attack almost impossible.
The hardness of the rock poses also some problems to the defender, as it was almost impossible for the Union soldiers to dig trenches for shelter on top of the ridges and hills, they had to rely on stone walls and improvised barricades.
On July 2 the Union Army had fortified Cemetary Ridge, formed by the broad Gettysburg Sill (760 m, almost 2.500 ft thick) and the Confederate Army gathered on Seminary Ridge, formed by a thinner diabase dike. The diabase plateau of Cemetary Ridge, large enough that all Union troops could be positioned there, and the impervious slopes proved to be decisive, as the attacking Confederate soldiers, despite all efforts and sacrifices, were unable to dislodge the Union troops.
July 3 would be the decisive day, as general Lee decided to risk a direct, full attack on Cemetary Ridge. However the sediments of the Gettysburg basin and the hornfels surrounding the Gettysburg Sill form only an apparent flat, as the tectonic tilted sediments and the slightly harder hornfels form a gently rising slope towards the diabase outcrops. A wide field without shelter, the slope slowing down the advancing soldiers, the Confederate infantry was an easy target for the Union artillery, firing from a raised position with increased range - the charge ended in a disaster for the Confederate troops.
July 4 finally general Robert E. Lee, taking advantage of the rainy weather, retires his troops towards the Potomac River - the Gettysburg Campaign was over, Civil War will however continue for another 2 years.
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