The Mesic Mixed Hardwood Forest is the main natural community in Rock Creek Park, with smaller patches of other natural communities intermixed like a mosaic.

The rolling slopes where this natural community grows typically have fairly deep acidic soils. They are loamy, meaning they are made of approximately equal proportions of silt, sand, and clay with some organic matter. The soils are mesic, that is, neither too wet nor too dry. Rainwater drains thoroughly, leaching away basic elements such as calcium and magnesium, creating acidic conditions.

In many parts of the park, these soils form over bedrock of the Laurel Formation, which consists of metamorphosed sedimentary rocks, or metasedimentary rocks. Ecobit: How metamorphic and igneous rocks were formed

Along the western tributaries to Rock Creek (particularly Broad Branch), slopes occupied by this natural community commonly have soils formed over Kensington Tonalite, which consists mostly of a metamorphosed rock that started out as an igneous rock similar to granite. Rocks in this formation have a mixture of light and dark minerals, giving them a salt-and-pepper look. Ecobit: “Rock Creek Granite”—the rock that built northwest Washington, D.C.