You don't swing in a hummock, you climb one. A hummock is a hill, a mound, or a ridge.
This word was once a nautical term describing a hill rising up on a coastline. A hummock is a perfect place for a lighthouse. Today it is either a rounded hill or an elevated place in an ice field. The origin of the word is unknown, but the ending -ock is a diminutive, meaning it gives the noun the sense of being small. That is why a hummock is a knoll and not a mountain.