South Mountain Reservation is located in the Newark Basin within the Piedmont Physiographic Province. The First and Second Watchung Mountains form the main Northeast-Southwest oriented ridges in the park. The West Branch of the Rahway River runs between these ridges and is dammed in several places to form a reservoir and several small ponds. It provides water for local wildlife and fish, as well as an abundance of aquatic macroinvertebrates. Bedrock, just below the mostly Boonton-type soil, can be seen as it emerges in many places. The famous Turtle Back Rock (D14) dates back 200 million years when lava cooled and cracked into polygons creating huge columns. What we see are the turtle-like markings at the top of the mostly buried columns.
A great variety of plant species are present. In general, the vegetation is of the mixed oak forest region of the eastern deciduous forests. Red and white oak and American beech in the well drained areas with maple-leaf viburnum, the dominant shrub. Winged euonymous, Japanese barberry, and Japanese knotwood are invasive understory shrubs in these areas. Chestnut oak is important on some of the drier slopes while the tulip tree thrives on the moister slopes. Red maple, spicebush and witchhazel shrubs are increasing in the poorly drained areas. Catbrier, jack-in-pulpit and false nettle are also common plants in these areas. Wildflowers struggle to exist among encroaching weeds, deer and invasive shrubs.
The Reservation’s forest is critical to dozens of species of migratory songbirds such as warblers, vireos, and thrushes. Species such as the Northern cardinal and the pileated woodpecker are year-round residents. Many other common species of wildlife find a home here including deer, raccoons, foxes, turtles, squirrels, rabbits, and wild turkey.