Saturday, October 4, 2008

Killer Limestones And The American Civil War

Okay, I guess the post title is a bit sensational, but an interesting study by Robert Whisonant and Judy Ehlen of Radford University in Virginia has explored the relationship of geological terrains and casualty rates at Antietam, one of the bloodiest battles of the American Civil war.

From Earth magazine summary:

Then, on Sept. 17, in the shadow of the Blue Ridge Mountains, the Battle of Antietam began. The armies blasted each other with gunfire from dawn until nearly dusk. That day proved to be the single bloodiest day of the American Civil War, with more than 23,000 men lying dead or wounded in the valley’s fields by nightfall.

But in addition to the bullets and cannonballs, soldiers in both armies had another common enemy that day: carbonate rock.

This was from an era where soldiers just used to line up on opposite sides and let go with their muskets and then charge at each other. The battle of Antietam in 1862 took place in the area surrounding the town of Sharpsburg, Maryland.

The fields are underlain by limestones and associated sedimentary rocks of Cambrian to lower Ordovician in age. Way back around 500 million years ago that area was part of a complex of Cambro-Ordovician marine sedimentary basins that stretched along the length of the now eastern U.S. Thousands of feet of carbonate sediments accumulated in these basins over time.

As with any depositional system the composition of the sediments varied over space and time. So there were depositional sub-centers where relatively pure carbonate sediments were being produced, other sub-centers which could be just a few 100 meters away accumulated a mix of carbonate and terrigenous clastic mud and sands. At places the original calcite sediments were replaced by the mineral dolomite.

Now these compositionally different sedimentary rocks respond differently to chemical weathering. Sediments composed of calcite (limestones) are most susceptible and over millions of years such areas will get worn down to relatively flat terrains. Areas with a mix of shale, limestone and dolomite respond differently. Limestone gets weathered faster than shale and dolomite and the results is a more uneven, undulating topography.

When the geologists mapped the casualty rates at Antietam to the underlying geology they found a correlation. The highest casualty rates were in those portions of the battlefield which were underlain by pure limestone and which had weathered down to a relatively flat terrain. On a plain, the opponent is in plain sight and there is nowhere to hide. The image below is a tilted view of an area known as Cornfield just north of Sharpsburg, where the casualty rate was especially high.

Here are a couple more images. The one below is a shaded relief map of the battlefield.

Source: Military geology of Antietam battlefield

The red circle in the upper part of the image is the Cornfield area. You can see the terrain is less dissected by streams and appears flat. The outlined area to the bottom of the image is more dissected by streams and shows a marked relief. This is the Burnside Bridge area which is a couple of miles from Cornfield and where soldiers could find cover in the deeply dissected gulley's and streams. Here casualty rates were three times lower than Cornfield.

The image below is a terrain profile of Cornfield and Burnside Bridge.

Source: Military geology of Antietam battlefield

Given time even granite terrains can wear down to a flat featureless plain. So the important factor here was not the composition of the rock by itself, but the difference in facies heterogeneity. Both sides during the Civil war used the terrain to their advantage. But studies which specifically try to draw out a connection of geology to a particular battle are rare and I enjoyed reading this one.

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