Manassas National Battlefield Park - Geology

Manassas National Battlefield Park by National Parks Service

Approximately 250 million years ago, the combined continents of Europe and Africa collided with North America, exerting massive pressure on what is today the eastern seaboard. This resulted in an enormous uplift of the entire region, called the Appalachian Orogeny. Pressure and heat over millions of years "cooked" the rock, folding, twisting and faulting it. Farther west the collision was gentler and resulted in less faulting and more wrinkling, creating ridges and valleys which became the Appalachian mountains.

Closer to the point of impact, the immense pressure turned igneous and sedimentary rock into metamorphic rock and broke it in numerous places, creating faults which were very susceptible to being worn away by wind and water. Over time these erosive forces wore the landscape down to nearly sea level. This is the Piedmont, where Manassas National Battlefield Park stands today.

The park is part of the Culpeper Basin, a distinctive geological province that is one of a series of Triassic-age trough depressions which border the eastern front of the Appalachian Mountains from Culpeper County into Maryland. The basin is characterized by low relief and gently rolling to nearly flat topography. It is composed of a matrix of sedimentary rocks such as siltstone and sandstone interspersed by intrusive dikes and sills of igneous diabase. Siltstone is a red-to purplish brown, micaceous rock that forms the parent material for most of the soil in the eastern half of the park. Diabase, which moved up through the siltstone during the upheaval of the Appalachian Orogeny, is much more extensive in the western half of the park. Diabase is a dense, medium-grained, dark gray to black mafic, igneous rock composed primarily of feldspar and pyroxene. It forms the parent material for much of the soil in the western half of the park. Where these two types of rocks came into contact, the heat of the emerging diabase melted the siltstone around it, creating narrow bands of metamorphic rock containing minerals such as epidote, cordierite, pyroxene, and garnet.

These two rock types in turn contribute to the makeup of the soil that overlays them. In general, soils derived from siltstone (79% of park soils) are strongly acidic, well-drained silt loams. Soils derived from diabase (19% of park soils) are generally loamy, rich in clay minerals, and tend to have subsoil hardpan, or fragipan, layers that limit soil permeability. The remaining 2% of soils in the park are generally alluvial, meaning they were carried and deposited by stream action.

Differences in soils in turn lead to differences in the vegetation that grows on them. Diabase soils, for example, support many rare grassland species that are similar to those found in the Midwestern prairies. The flat, poorly drained nature of some of the park's soils support unique communities of oak species that are far less common in other areas of the Piedmont. As you can see, Manassas National Battlefield Park is part of an extremely dynamic geologic region that has helped to create its unique landscape.

-Information for this article was obtained by Virginia Department of Conservation and Recreation reports and from Watching Nature , by Mark S. Garland.