* Swarm of bees: The noun is swarm. "Of bees" is a prepositional phrase that modifies the noun "swarm."
* Heap of stones: The noun is heap. "Of stones" is a prepositional phrase that modifies the noun "heap."
Key point: In both cases, the noun is the word that names the thing or group of things being referred to. The prepositional phrases provide additional information about the nouns.