Exposed bedrock with rock polypody.
Photographer: Milo Pyne, courtesy of NatureServeThe setting for the Subcalcareous Cliff and Rock Outcrop is exposed bedrock cliffs overlooking the Potomac. There are two examples of this natural community in Harpers Ferry National Historical Park: one is the famous Maryland Heights cliffs beneath the Maryland Heights overlook, and the other is on the eastern side of Short Hill. The cliffs at Maryland Heights face southwest, and the cliffs at Short Hill face northeast. (Ecobit: How Topography Creates Microclimates)

Although the cliffs are nearly vertical, soil can accumulate in cracks or in moss on ledges, allowing plants to take root.

A Geologic Puzzle

The term “subcalcareous” in the natural community’s name refers to the fact that the soil (what little there is) contains more calcium than is usual for Harpers Ferry's more acidic soils. (Ecobit: Defining Terms—Basic and Acidic) Typically, "basic" elements such as calcium come from minerals in the underlying bedrock.

At Short Hill, the bedrock is the Catoctin Formation, which is metamorphosed basalt, and basalt does contains minerals rich in calcium and magnesium. However, the metamorphosed sediments (Harpers Formation) and the quartzite (Weverton Formation) mapped at Maryland Heights are not known to be rich in calcium or magnesium. That’s a puzzle. What could be the source for these elements?

It may be that these rock layers were originally overlain by calcium carbonate deposits, such as the Tomstown and Antietam formations. In that case, calcium could have leached down into the Harpers and Weverton layers underneath (Charles A. Davis, 2014, “Survey of Rare, Threatened, and Endangered Plants of the Maryland Heights Cliff”). There may be other processes that created calcium-rich deposits in the cliff’s cracks and fissures. (Bird droppings?) In any case, the plant species found here tell us that there is a source of calcium and magnesium.