Daily Heights pointed me in the direction of an interesting web site called, Forgotten New York, which includes a history of Prospect Park. Says Daily Heights:  "It is full of delicious tidbits about Revolutionary landmarks, the
Quaker graveyard, that gorgeous Horse Tamers sculpture, why the Vale of
Cashmere has that funny name (and why you should stay away from it). In
particular, I appreciated his explanation of those annoying hexagonal sidewalks:"

"Prospect Park, and indeed most New York City parks, employ special
sidewalks on their exteriors and on some park walks consisting of
interlocking hexagonal blocks, that can be hard to walk on at times.
Most likely, they are there to accommodate tree roots; when roots
interact with the usual concrete slab sidewalks, the sidewalks lose the
battle and split, making for dangerous walking conditions."

LINK: Secret Prospect Park [Forgotten NY]
Thanks for the tip: satanslaundromat