Gettysburg National Military Park

July 02, 2006

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Provided by: Dave Kerr
Summary authors & editors: Dave Kerr

The diabase outcrop of Devil’s Den at Gettysburg National Military Park in Pennsylvania is among the best known mass of boulders in the United States. Here, two great armies clashed from July 1st through July 3 in 1863. The geology of the landscape played a significant role in the outcome of the battle of Gettysburg. Dating from the early Jurassic period (approximately 200 million years ago) and the result of geologic rift, these outcrops of the York Haven diabase are highly resistant to weathering and extremely durable. These qualities made this igneous rock, composed of feldspar and pyroxene, economically desirable for monuments and crushed stone.

During the battle on July 2nd, the Union Army’s position at Devil’s Den was captured by the Confederates and used by sharpshooters to harass Union forces on Little Round Top, visible in the upper right. This high ground was controlled by the Union Army and extended to Culp’s Hill in a fishhook shape. The diabase on Little Round Top was difficult to entrench, and the soldiers were forced to crouch behind boulders or stone walls for protection. Despite brave and determined assaults against Little Round Top late on the 2nd by Confederates of Hood’s Division, the Union Army’s left flank was held by the stubborn courage of the men of the 20th Maine. This set the stage for General Robert E. Lee’s gamble of a frontal attack against the Union center the following day. Photo taken on May 6, 2006.

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