Right near the start of the route, coming from the Marvelous Bridge side, is a Pokefan. He's got a Growlithe L60, a Houndour L60, and a Stoutland L60. Inside of the nearby RV, a Scientist in there will trade you a Rotom for a Ditto. You can find Ditto inside of the forested part of the Giant Chasm, whereas Rotom can only be gotten here, so it is a good trade.
You can now either climb the stairs and cross the bridge, or you can head down the cliff by walking down the stairs in the southern part of the route. It's quicker to get to Black City or White Forest by crossing the bridge, but you don't want to miss out on the TM down there, so I'll cover that part.
Down the stairs is a Backpacker who has a Seedot L60, a Cacnea L60, and a Croagunk L60. Beat him, then hop down the first ledge, although before jumping down the second one, make sure you pick up the hidden HP Up. The next Backpacker down there has a Poochyena L60, a Torkoal L60, and a Cubone L60. To the west of her, by the ledges, are some steps that leave over to a Max Potion.
Go on over to the boulder nearby the Backpacker and push it over to the west three times, so it is one space away from the wall, and then walk over to the other side and push it twice to the north, then one more time to the right so it is in the hole. Up there, you can find the thick dark grass, as well as a hidden Ultra Ball.
Back over to the east, there's one more boulder you can push to reach a valuable TM. Push it to the south three times, so it is one away from the southern wall, and then push it over to the right just once. Then, push it south one and east one to fill the hole. Do that and you'll be rewarded with TM26 (Earthquake), which is one of the strongest attacks in the game. It's a powerful 100 power Physical-based Ground-type attack that hits all adjacent Pokemon.
Now, head north and up the steps to reach the other side of the bridge. You'll have to fight against a Pokefan first, and she uses a Meowth L60, a Glameow L60, and a Liepard L60. To the east is Black City (Black 2) or White Forest (White 2). Before going there, though, head over to the west and across the bridge until you find the Poke Transfer Lab, as well as a Scientist outside who uses a Gulpin L60, a Garbodor L60, and a Magnezone L60.
Further west, you'll find the start of the route. No need to go back there. Instead, you can stop by the Poke Transfer Lab right in front of you if you want to transfer your Pokemon from the previous generation of games. Otherwise, just head over to the east to reach Black City or White Forest.