I was at the north end of Central Park recently and took the picture above. That’s the demising wall separating the park from 110th Street (AKA Central Park North), as seen from a footpath just inside the park. That wall was built in the late 1860s, shortly after the park itself was complete enough to be opened to the public.
The wall is clearly divided into two portions: a top of constant height with tight joints and a deliberately-roughened (probably bush-hammered) surface, and a bottom of varying height, with rounded faces and deep joints. It doesn’t appear to be the same stone: the upper portion is reportedly sandstone (not, fortunately, brownstone) and the lower portion appears to be Manhattan schist. Some of the lower stones (like the one just about dead center in the photo, below and to the left of the white truck) have drill marks from quarrying, which is not something you’d expect to see on sandstone.
The path I was walking on is below the level of the adjacent sidewalk on the street. As a matter of fact, the dividing line between the upper and lower portions of the wall is right about where sidewalk grade is located. So it sure seems like the more durable schist was used for the foundation and retaining-wall portion of the wall and the sandstone for the free-standing screen portion of the wall. The idea of matching stone to its intended use has a very long history and was certainly common in New York in the nineteenth century.
Landscapes change over time, even ones as tightly controlled as that of Central Park. There is no guaranty that the footpath was always lower than the street by the same amount it is today. There are a lot of mature trees in this area, so grade elevation has not changed much in recent years, but maybe it was higher originally. Probably not, though, as there are plenty of other places in the park where the grade is lower than the surrounding streets. It’s important to remember that, while the park is an artificial landscape that was heavily graded to match the Olmsted and Vaux design, the street areas outside the park are far more artificial, and that hills were removed and valleys filled in to create a more uniform street grade. Where the park is lower than the surrounding streets, it probably is closer to the original grade than the streets are.