This section of the wall (Street View) is made in large part of grave markers and other cemetery stones! You can see that some are lying flat on their sides, while others are standing upright. Many of them look broken, and perhaps the rest were surplus — not a bad way to get some use out of what would otherwise just be a bunch of junk.
Or as Mitch Waxman puts it, much more colorfully:
As one nears Laurel Hill Blvd. and the stature of the masonry wall shrinks back to a human scale, a curious heterogeneousness in its composition is noticed. Suddenly granite and "finishing marble" is noticed. . . . Proceeding up the block, certain familiar shapes become recognizable in the wall, and a cold dread is realized. Tombstones. They used tombstones to make this part of the wall.