Hamilton Preserve is considered by many to support the world’s largest population of the federally threatened and state endangered plant, Knieskern’s beaked rush. Seemingly unremarkable, the small 10-12 inch high plant in a family of plants known as rushes, could be overlooked as just another grass-like weed. However, this small rush has come to skillfully select and adapt to disturbed areas with wet sand as well as the Pinelands naturally occurring vernal pond habitats. Sometimes known in this region as “pingos” or coastal plain ponds, these vernal ponds are small and often very round in shape. These ponds are filled in spring but dry up each summer and become grassy openings in an otherwise forested landscape. This “drying out” period is key not only for the Knieskern’s beaked rush, but also for the Pine Barrens treefrog that depends on these ponds for breeding. In spring, the treefrog's eggs cling to sticks or other debris in the water and develop into tadpoles, then leave the pond as frogs before the pond totally dries. The annual drying eliminates any fish that might pray on the eggs or tadpoles. But deep ponds with fish are also part of the landscape. Years ago, many areas of this preserve had been mined for sand. Now left behind are multiple, year-round ponds which add diversity to the habitat. Pitch pine uplands and lowlands, Atlantic white cedar bogs and shrub oak/sheep laurel thickets are some of the more common forested communities at Hamilton Preserve. The preserve contains the headwaters of the Gravelly Run, a tributary to the Great Egg Harbor River, and part of the National Wild and Scenic River System.