Anthracite miners at war: military geology and engineering of the Petersburg mine—June-July, 1864
Inners, Jon D. (retired), Pennsylvania Geological Survey, P.O. Box 8453, Harrisburg, PA 17105, and Inners, Brant E., CUNY Staten Island, 512 Arlene St., Staten Island, NY 10314.
At 4:44 AM on July 30, 1864, Union forces detonated a mine containing four tons of black powder beneath a Confederate fort on the Petersburg, VA, front, blasting a hole 170 feet long, 80 feet wide, and 30 feet deep in the rebel line. The mine had been dug by men of the 48th Pennsylvania Volunteer Regiment (IX Corps) from Schuylkill County—many of whom were peacetime anthracite miners—and had been designed by Colonel Henry Pleasants, a professional mining engineer who worked for the Philadelphia and Reading Coal and Iron Co. before (and after) the war. That the ensuing "Battle of the Crater" unfolded as perhaps the most infamous fiasco of the American Civil War is no reflection on the yeoman work of these citizen soldiers.
The Petersburg mine was excavated though Coastal-Plain sediments underlying the "Sunderland terrace" about a mile southeast of the Fall-Line city of Petersburg. The terrace is capped by the Tertiary-age Bacons Castle Formation (fluvial-deltaic sand and gravel), but the mine was mainly within the underlying Chesapeake Group (also Tertiary in age), particularly in the Eastover Formation (marine clay and silt) (C. R. Berquist, pers. comm., 1989). The portal was located near the base of a gentle 30-foot-high colluvial slope on the west side of Poor Creek, and the mine ran in a west-southwest direction from there. A bed of stiff clay ("marl") encountered about 200 feet in from the portal forced the miners to ramp the tunnel upward for a short distance so that the powder magazines ended up only 20 feet below the Confederate fort.
The main mine-tunnel was 510 feet long, with two 38-foot lateral galleries at the far end—probably the longest mine ever attempted to that time. Roof support was provided by timbers scavenged from a nearby bridge and sawmill. To ventilate the mine, Pleasants designed an ingeniously simple system, consisting of a furnace, a wooden duct, and a canvas partition, to draw stale air away from the working face and fresh air in from the outside.
Once the mine was "sprung" the main geologic influence on the outcome of the battle was the existence of a deep natural ravine to the north and west of the Crater, which allowed Confederate General William Mahone to launch a surprise counterattack that stopped the Union advance and consigned thousands of enemy soldiers to death, mutilation, or capture in the "horrid pit."
Oral paper presented at the 49th Annual Meeting of the Southeastern Section Geological Society of America, Charleston, SC, March 24, 2000.
Inners, J. D., and Inners, B. E., 2000, Anthracite miners at war; military geology and engineering of the Petersburg mine—June-July, 1864: Geological Society of America Abstracts with Programs, v. 32, no. 2, p. 27.