Oak - Beech / Heath Forest on a steep hillside, featuring mountain laurel.
Photographer: Ery Largay, courtesy of NatureServe
Shared under Creative Commons license BY

A thin veneer of soil covers steeply sloping bedrock in the Oak – Beech / Heath Forest. The soil is mineral soil, meaning it’s composed primarily of particles of weathered rock (sand, silt, and clay) rather than decomposed leaf or animal matter. It’s also acidic, due both to the parent rock, and to the fact that rainwater running over the slopes leaches away basic elements such as calcium and magnesium. The acidity makes the soil relatively infertile. Although excess rainwater drains off rapidly, some moisture is retained by duff (decaying leaves and organic matter) on top of the soil, and patches of moss. The nearby stream keeps air moist.

The steep slopes are characterized by small to large outcrops of bedrock—at Rock Creek Park, typically Laurel Formation, or less frequently Kensington Tonalite. Because of a number of geologic faults along Rock Creek, the rock exposed on some of these bluffs is mylonite—a rock that’s been stretched like taffy by prehistoric fault motion. Angular outcrops and broken chunks of white quartz veins are also common, lending even more acidity to the environment. Ecobit: For Fault-Finders in Washington