Tripod Rock

Tripod Rock is a balancing rock, or perched boulder, located in Kinnelon, New Jersey in the Pyramid Mountain Natural Historic Area. This multi-ton Precambrian gneiss boulder, located near the edge of a long ridge, is balanced on three smaller boulders. Tripod Rock is roughly 6 m (20 ft) long, 3 m (9.8 ft) wide, and 2.5 m (8 ft 2 in) high, weighing approximately 127 tonnes (280,000 lb). A triangular crest runs the length of its top. The boulder is balanced on three smaller stones roughly 1 m (3 ft 3 in) in diameter that raise it above the bedrock by about 0.5 m (1 ft 8 in) at its lowest point. The point of contact between the boulder and its support stones forms an approximate 3-4-5 triangle.

Tripod Rock from the east (video)

About 12 m (39 ft) northwest of Tripod Rock are three boulders that appear to form a triangle. The two larger boulders are balanced partially on smaller stones (see diagram). The apex rock is about 3 m (9.8 ft) away to the northeast. Sighting from a bedrock outcrop near Tripod Rock through the two large boulders, the apex of the triangular configuration points to the next ridge on the horizon to the west. This line of sight is toward the setting point of the sun on the summer solstice. During the early 1980s, Tripod Rock and the stones near it were surveyed. The place on the ridge where the sun sets on the summer solstice was visible due to gypsy moth defoliation.[1] A large boulder was visible on the ridge at the place of the summer solstice sunset. Within a year of the survey, a home was constructed on that site and the boulder was displaced by earth-moving machinery. The summer solstice sunset is presently observable near the edge of that residence, and is only partially visible.

Boulders and outcrops near Tripod Rock, including summer solstice alignment

Two smaller perched boulders are located northeast of Tripod Rock (not shown on diagram) that measure approximately 1–3 m (3 ft 3 in–9 ft 10 in) in diameter. No specific calendar alignment has been noted. Tripod Rock stands on a ridge overlooking a long valley where a massive glacial erratic named Bear Rock is located near a brook. Bear Rock was thoroughly excavated for archaeological artifacts in the 19th century, some of which are reported to be owned by museums. Bear Rock has a large overhang making it a possible rock shelter.

While there is no evidence that Tripod Rock was ever used as a solar observatory, it could function as one if a small portion of the ridge across the valley were kept clear. Tripod Rock is presumed to be a naturally deposited, though oddly perched, glacial erratic.