The reservation covers an area of 1,945 acres (787 ha), as it is bounded by the city of Summit, the borough of Mountainside, and the townships of Berkeley Heights, Scotch Plains, and Springfield.

The Blue Brook has carved a valley in the Reservation between the 1st and 2nd Watchung Mountains along the strike of the less weathering-resistant red beds of the early Jurassic Feltville Formation.

[2][7] Evidence of the Lenape presence in the Watchungs can be seen in numerous camp sites that have been uncovered, mainly along the rivers coursing through mountains and in the small caves abundant in the volcanic rock.

It is thought the Lenape favored the Watchungs for their profusion of natural resources, including abundant freshwater rivers and streams, a variety of forests, and plentiful fish and game.

In 1882, Warren Ackerman bought the property and converted it to a summer resort, called Glenside Park, offering golf, boating, and horseback riding.

A local civil engineer and travel enthusiast named Edward J. Grassmann purchased a number of properties to serve as a club, which he eventually sold to the Union County Parks Commission.

Grassmann was a lifelong Elizabeth resident who made his fortune in land surveying and kaolin processing; the trust that bears his name is still in existence today.

"[13] It was Grassmann who apparently persuaded celebrated Nicaraguan/Mexican artist Roberto de la Selva to paint themed murals throughout the first floor interior of the Mexican Cottage.

In the late 1920s, during his first trip to the United States, de la Selva spent months in rural Union County painting the murals, which depict native Mexicans at work, play and worship, including statues of both ancient gods and the Virgin Mary.

[16] Soon after the Union County Park System was formed in 1921, purchasing land to create the Rahway River Parkway to the east, this area was incorporated into the Watchung Reservation.

[25][26] Climbers within the state have been drawn to the bouldering opportunities on the cliffs along the brook and Diamond Hill Road at the Watchung's western boundary, although climbing is technically prohibited by the county.

[30] The clearing was created in the late 1950s when, against strong local opposition, the U.S. Army built the Cold War defensive Nike missile base here.

The Army Corps of Engineers wrote in 1981: Blue Spotted Salamander, Ambystoma leterale, classified as an endangered species by the State of New Jersey has been recorded as having occurred in the Watchung Reservation (U.S.

Zappalorti (1978) also did not find any, but reports sightings in 1979.In its 1981 environmental impact report on a flood control project,[39] Army Corps of Engineers discussed the unique natural spring environment in the Reservation, which provided habitat for six species of salamander: Scattered along the northwest slopes above Blue Brook in the Watchung Reservation, from the "Glen" to Seeley's Pond, are numerous springs that are of apparent great antiquity.

There have also been reports of sightings of the Blue-spotted salamander in the springs of Watchung Reservation.In spite of heavy local public protest, higher levels of government have taken land in the reservation twice for their own purposes.

[41][25] The Sierra Club,[42] other environmentalists, and hikers claimed that the county had failed to behave transparently in working with a group of cyclists and bicycle store owners in pushing a plan to carve 13.5 new miles of bike-only trail in untouched forest in the Reservation.

[43] They asserted that no environmental impact studies had been performed on the Reservation itself, that hiking advocates like the New York–New Jersey Trail Conference (NYNJTC) had not been consulted, that no attention had been paid to the Olmsted landscape architectural legacy of the reserve, and that the county had gone straight to hiring an engineering consultant firm, Parlin-based CME Associates,[44] to draft a master plan to insert the bike-only trails.

[45][46][47] In response, one Union County bicycle store owner dismissed concerns as biased and misguided hysteria, noting the many health benefits of mountain biking.