Perhaps the most unique rocks in
These eruptions did not occur all at once, but over a period of several million years. Discrete eruptions produced individual flows from 20’ to over 100’ thick, which spread over the landscape and cooled before the next eruption. These distinct lava flows still affect the landscape of Shenandoah today, as the individual layers create flat “benches” and sheer cliffs that give peaks such as Stony Man a noticeably staircase-like texture. Big Meadows, a broad, near-flat area at high elevation, is located on the surface of one of these lava flows.
The lavas were originally composed of basalt, a black volcanic rock similar to those found in
Shenandoah’s greenstone lava flows can have many different appearances. They often appear as jagged cliffs, or steep fields of angular grey boulders. In general, the rocks are very fine-grained, so that individual minerals can rarely be seen. On a freshly broken face, the rocks look dark grey to dark green, but more weathered and eroded surfaces often appear light grey or rust-red. In many cases the rocks appear to contain many thin layers tilted at a sharp angle, producing an uneven, jagged look; this is a metamorphic texture called “cleavage”, formed by deformation and the growth of the mineral chlorite.
The greenstones also contain several very unique and interesting geologic features. The first is columnar jointing, a fracture pattern that forms as liquid basalt flows cool and solidify. Like most materials, lava contracts as it cools, and under the right conditions will form very angular, polygonal cracks similar to those found in drying mud. These cracks can extend vertically for many tens of feet, and produce a structure that looks like long, polygonal columns of rock. The best example of these along