Although the park does not contain volcanoes, canyons, or other such grandiose geologic formations, its topography and geologic features are equally important in the influence that they had on the historic Civil War battle. Approximately 180 million years ago during the late Triassic Period, the Gettysburg Formation comprising sandstones, siltstones, and shales was deposited in a large carved out basin in the
The intrusion dikes are composed of very fine-grained, dense diabase rock very resistant to weathering. The composition of these rocks indicates that the molten masses cooled very rapidly. The
Geology was important in the outcome of the battle of
The boulders scattered across the landscape and also the rocks of Devil’s Den, provided cover and strategic defensive positions for the soldiers. Thin soil on many of the sides and tops of hills also made it almost impossible for Union soldiers to entrench themselves. The resistant diabase bedrock was so close to the surface on these hills that the troops were unable to “dig in.” They had to rely on existing stonewalls, scattered boulders, and outcrops of rock for protection. Because of the Union army’s inability to entrench its position, it suffered heavy losses of 23,000 men, while the attacking Confederates lost a little more than 28,000.