From Bulbapedia, the community-driven Pokémon encyclopedia.
The Charicific Valley Natural Reserve, better known as simply the Charicific Valley (Japanese: リザフィックバレー Lizafic Valley), is a location in the Pokémon anime.
Located just outside of Violet City, the Charicific Valley is a training ground for Charizard so they can become stronger. It is distinguishable by its rock formation in the shape of a giant Charizard.
The only known human who lives in the valley is Liza, who, along with her Charizard, Charla, protects the valley from danger.
Ash left his Charizard here to train when he was heading through the Johto region. Noticing how much his Charizard enjoyed spending time in the valley, and noticing that it was one of the weaker Charizard there, Ash decided to leave it in the valley so it could train. Since Charizard has been in training, it has become very powerful, learning many new attacks including DragonBreath, Overheat, Steel Wing, Wing Attack, Slash and Dragon Tail. It has also formed a romance with Charla.
The valley was seen in a flashback in The Fires of a Red-Hot Reunion! by Ash, who recalled how he left Charizard.
Pokémon seen in Charicific Valley