Tour Isabelle, Natural arch in Plateau-des-Petites-Roches, France

Tour Isabelle is a limestone arch in the Aulp du Seuil cirque of the Chartreuse mountains, located at about 1790 meters elevation. The formation displays two stacked openings that define this section of the eastern mountainside.

The arch stayed out of guidebooks until late May 2005, when Pascal Sombardier came across it by chance while exploring the Chartreuse massif for his book Chartreuse inédite: Itinéraires insolites. He named it Tour Percée, while the landowner called it Tour Isabelle after his second wife. Photographs of the double arch appeared on the book's cover, published in 2006.

The arch has two names: Pascal Sombardier, who found it in May 2005, called it Tour Percée, while the landowner named it Tour Isabelle after his second wife. The name Tour Percée refers to the pierced rock tower.

The arch stands on private land, and the owner and tenant of the estate dispute free access to it. Its approach is difficult and dangerous, which is why even recent guidebooks rarely mention it. The site lies at about 1,790 m on the eastern rim of the Chartreuse massif, above the Grésivaudan and opposite the Belledonne chain, in the Hauts de Chartreuse national nature reserve.

What sets this formation apart is its double-arch structure with two stacked openings, the lower passage spanning around 30 meters wide. This scale ranks it among the largest natural arches found in the Alps.