If it's the set of small rocks I'm thinking of, those are actually "seal rocks" that were erected by one legendary Pokémon to protect its friends from humans. In order to get past the rocks, you must first earn that legendary Pokémon's trust by catching it, and then it will automatically move the rocks. There is another identical set of rocks deep within Pinwheel Forest that will also be moved at this time.
This legendary Pokémon can be found at an optional cave called Mistralton Cave. Go to the season research lab on Route 6 and Surf eastward along the river above the lab, and then you'll see the cave almost immediately. Be sure to bring Pokémon with Strength and Flash, as well as plenty of Poké Balls and Pokémon that can weaken the legendary Pokémon for capture. After you catch it, you can catch another Pokémon at Victory Road behind those rocks and a third Pokémon behind the rocks in Pinwheel Forest.