Before Frederick Law Olmstead was commissioned to turn it into a park in 1857, the 840 landscaped acres now in downtown New York City were wet meadows, lagoons of human and animal sewage, a few small inholdings and tiny "villages" owned by untamed Irish and free slaves, and solid rock outcroppings ground smooth by glaciers.
Though Central Park was to be America's first *landscaped* public park, the land was so barren and fouled that 500,000 cubic feet of topsoil had to be imported from New Jersey right from the start. Modern estimates put the amount of New Jersey top soil now in the park at around 10 million cubic yards.
Central Park started off with a grand plan, but after initial clearance, construction, and planting, City Hall corruption siphoned off all the maintenance money and the park went into a long decline. By the mid-1930s, Central Park was shabby and uninviting and New York City Mayor Fiorello La Guardia instructed urban planner Robert Moses to turn it around. He did so by replanting the lawns and flowers beds, cutting out and replacing dead trees and bushes, cleaning and repairing bridges and walls, installing new playgrounds, ballparks, and a skating rink, and putting in new sculptures, a children's carousel, and a renovated zoo. Central Park became a destination once again.
The massive and rather dramatic rocks and boulders that are strewn across Central Park are schist and gneiss remnants of glaciers which scoured North America and then retreated about 12,000 years ago. One of the more famous big rocks is Rat Rock where urban boulder climbers practice their passion.
New York City is mostly built on solid rock -- one reason the buildings can rise so tall above an island riddled with subways, tunnels, and basements.