‘Fascinating Geology’ Supports Hallowed Ground

American Association of Petroleum Geologists (AAPG)

There are almost 50,000 souls at Gettysburg, and part of this nation’s DNA is in that soil. It is clearly a place, rich in history, tragedy and politics.

And geology.

For purposes here, as we approach the sesquicentennial of the battle that gave this land its lasting place in history, that is the topic – not so much about the spirits that inhabit the place, but the place itself.

Gettysburg is not only a battleground, not only a cemetery, but also was a blueprint for war.

“Few battles in recorded military history exhibit the relationship of topography and geology to the ultimate outcome of the fighting as clearly and dramatically as the Civil War battle of Gettysburg.”

That’s from AAPG member John Harper, chief of the Geologic Resources Division with the Pennsylvania Geological Survey, who will be part of a group that leads a field trip to the historical site as part of this year’s AAPG Annual Convention and Exhibition in Pittsburgh (and again during the actual sesquicentennial July 1-3) called “Rifts, Diabase and the Topographic ‘Fishhook’: Terrain and Military Geology of the Battle of Gettysburg.”

“The geology is fascinating,” he says, “one of the rare areas of Pennsylvania with rocks less than 250 million years old.”

Harper says the terrain, part of the break-up of Pangaea in the Triassic/Jurassic era, played a key role in what leaders of both armies did during the battle.

“From McPherson’s Ridge to Seminary Ridge to Devil’s Den to the ‘topographic fishhook’ (all in the Mesozoic Gettysburg rift basin), the tactics of the competing commanders – Meade for the Union and Lee for the Confederacy – can be followed and rationalized with the use of a geologic map.”

Geologic Controls

Harper says one can plot the arrows to show troop movements from both sides that coincide with the vicissitudes of a land where almost as many Americans died in three days of fighting in south central Pennsylvania than did in 18 years of direct U.S. involvement in Southeast Asia.

A place like Gettysburg, then, should be seen from all disciplines – cultural, historical and, yes, even geological.

Jon Inners, who also is with the Pennsylvania Survey, explains, “The entire Gettysburg campaign from mid-June to mid-July was controlled by the Blue Ridge and Piedmont terrain of Virginia, Maryland and Pennsylvania.”

He says that General Lee knew the area well.

“Lee used the Blue Ridge (called ‘South Mountain’ in Maryland and Pennsylvania) to shield his troops movements to Gettysburg on the way north, then used the mountains equally well to protect his army when he withdrew after the battle.”

The course of the entire three-day battle, Inners reiterates, was directly controlled by this topography, with Lee driving the Union forces from the low hills west and northwest of town so they took up a final defensive position on the “topographic fishhook” of diabase ridges east and southeast of town.

Day by Day

The trip will encompass the highlights of what happened once he got there.

♦ Day One – Lee’s approach to Gettysburg from the west.

“He used the deep gaps in the mountain range (some of which are formed directly along large east-west faults), Inners said, “then retreated through these same gaps on July 4-5.”

♦ Day 2 – The fights at Devil’s Den, Little Round Top, the Peach Orchard, southern Cemetery Ridge, Cemetery Hill and Culp’s Hill, which Inners says were “fierce and indecisive.”

♦ Day 3 – Cemetery Ridge, the final decisive engagement, including the infamous “Pickett’s Charge.”

Specifically, during that last day, Inners explains how Meade used the terrain quite effectively, and how General Daniel Sickles almost “screwed things up” for him by advancing west (sans orders) from a low spot on Cemetery Ridge to the Peach Orchard, a spot from which the Confederates nearly crushed them.

“In short, the Confederates were on the attack the entire battle, and the Union on the defensive,” Inners said.

Inners and Harper are not steeped in wartime tactics, but both know had things turned over differently at Gettysburg, things would have turned out differently for America.

“It certainly would have made things much more difficult to arrive at a Union victory,” Inners said. “But, thank goodness, that’s how it all turned out!”

What a Trip!

As for the trip itself, participants will spend two days traversing and exploring the sacred ground, and Harper says as a bonus, a stop will be made at the stone bridge over Plus Run and the base of the Big Round Top to view the Triassic-age dinosaur tracks.

“All of the folks (and this includes geologists and paleontologists) who will be leading the trip, other than me, are big Civil War history buffs,” Harper said.

One more thing: he says the weather is usually unpredictable, in that part of Pennsylvania, and participants should be on the lookout for ticks and poison ivy.

Why is that important?

Inners isn’t so sure.

“Don’t let Harper’s answer bother you too much,” Inners laughs. “May is certainly one of the nicest months in south-central Pennsylvania. The battlefield is quite open, and there really isn’t much high grass where ticks would be a problem. If the weather is nice, Gettysburg is a beautiful place!”

So, no need to worry?

“Best to be safe, however, and plan for them.”

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