After you visit the Fossil Lab in Ambrette Town, you learn that you need ride a Rhyhorn to Glittering Cave.
Ride the Rhyhorn
Go down the stairs to find a Rhyhorn. Walk to it from the side to get onto its back. Now you can walk over the rocky path. While you ride around on the Rhyhorn, you may encounter Helioptile, Sandile, and Hippopotas.
When you reach two boulders, stand in front of one and press A. Rhyhorn will headbutt the boulder and get rid of it.
Up ahead, there is a boulder blocking some stairs up to an item. Press A to headbutt the rock, then press B to get off of the Rhyhorn so you can go up the stairs and get the X Defense there.
Farther ahead, there is a fork in the path. Go to the right side where you can see an item. Headbutt the rock, then press B to get off. Walk up the stairs to get the Paralyze Heal. South of where you got the item, there is a narrow ledge that you can walk along to reach a Fire Stone.
Get back on Rhyhorn and go along the other side of the fork in the road. Headbutt the three boulders in the way. You'll see some people to the left, but first go to the right and get off of Rhyhorn when you are near the steps. Go up and go south to find a Dusk Ball.
If you go to the left, you can go up two flights of stairs. When you reach the end, the trainer on the ledge across from you will ask if you want to have a Sky Battle. To do a Sky Battle, at least one of your pokémon has to have the Flying type. Only pokémon with the Flying type can participate in the battle. Sky Trainer Orion has a level 21 Ledian.
Now ride the Rhyhorn to the area where you saw other people. Talk to the girl in red to get your pokémon all healed up.
Go up the stairs and into the cave to reach Glittering Cave.