Before you leave for new horizons, head back to Verdanturf Town and go inside the Rusturf Tunnel. There, go north and meet up with the couple divided by the boulders. (What, they can’t climb over them?)
Now that you can use Rock Smash outside of battle, opt to break the rocks for them and, in return, you’ll get some Aggronite , which will let Aggron Mega Evolve later on!
After that’s finally been resolved, return east to Mauville, then take the north exit onto Route 111 .