GEOLOGIC CONDITIONS AFFECTING TUNNEL CONSTRUCTION AT THE 1864 BATTLE OF THE CRATER, PETERSBURG, VIRGINIA

The Virginia Division of Geology and Mineral Resources have been creating detailed geologic maps in eastern Virginia over the past decade and Colonial National Historical Park and parts of the Richmond National Battlefield Park have been included in this work. The mapping has been partly supported by the USGS/AASG Statemap Program. Some initial work was recently completed in Petersburg National Battlefield.

On July 30, 150 years ago, Union troops with Pennsylvania coal mine experience had completed a tunnel underneath Confederate defenses at Petersburg, Virginia and detonated 8000 pounds of gunpowder inside the end of the tunnel. This resulted in a massive crater, great loss of lives, and a disastrous outcome for the Union. We placed three auger borings adjacent to this tunnel to define stratigraphy, understand and explain problems with its construction. Initial digging of the tunnel began in the wet shelly sands of the Pliocene marine Yorktown Formation. Collapse and other difficult mining conditions forced the decision to angle the tunnel upwards and hopefully into more stable material. The mining then encountered lenticular-bedded sandy clays of the Late Pliocene marginal-marine Cold Harbor Formation, material more competent than the Yorktown sediments, and the tunnel was successfully completed. This 500-foot long tunnel to penetrate fortified positions was not unique to standoff warfare that occurred through the ages though it was exceptionally long; geologic conditions in the Petersburg area permitted this strategy. Confederate forces responded in less than a week by placing powder in tunnels they had countermined under Union lines a short distance to the north of “The Crater” and detonating smaller-volume explosives with mediocre results.

Recent auger drilling coincides with the 150^th anniversary of tunnel construction and the battle of “The Crater”. Now, with partial funding from the National Park Service, we are expanding geologic mapping into Petersburg National Battlefield. The anticipated new geologic mapping information should support resource management for the Park and is expected to enhance interpretive programs for visitor experiences.