These are the actual slabs in which Dexter Marsh first noticed "bird tracks" while laying a sidewalk for the town of Greenfield, Massachusetts, in the late winter of 1835. The left slab holds the mold, meaning the original footprints. The slab on the right has the cast, meaning the material that filled in the original tracks once they had dried enough to hold their form. Marsh recognized a pattern in the tracks that indicated that they were made by a living creature. Their shape reminded him of bird tracks he would have seen commonly around him.
Amherst College eventually bought these slabs. They are now on display at the campus's Beneski Museum of Natural History.