Image credit: David George
The Tour Percée double arch is a double natural arch, located in the Parc Naturel Régional de la Chartreuse, Chartreuse Mountains, France.
Located in a very remote area, with a difficult and dangerous access, very few guide books, even the most recent ones, mention its existence
“The longest natural erosion arch in the Alps (at 32 metres, and a double one at that) was only discovered in 2005, in the Chartreuse mountain chain above Grenoble, at the southern tip of the Jura mountains (part of the Pre-Alps), long famed for the bittersweet herby alcohol produced by the Carthusian monks (reputedly some of the most austere, meditating in their isolated eyrie, where they have been based since 1084). The find was made by a hiker who was researching a book on the hidden unknown treasures of the range, later published as Chartreuse inédite : Itinéraires insolites. The name is somewhat unoriginal, meaning the pierced tower, though the feature is also known as the Tour Isabelle. Access is difficult.
The rocks here are limestones, riddled with complex karstic cave systems eaten out by slightly acidic rainwater. They were once oozes on the bottom of the Tethys Ocean, which started its long existence when Pangaea started rifting apart, and whose last dying gasps as the gap is closed again by Africa’s collision with Europe is now known as the Mediterranean sea. They were pushed upwards after the sediments tuned into rock (a process called diagenesis) as the inexorable grind of continent against continent squeezed up the intervening rocks like toothpaste and a rumpled tablecloth into a mighty mountain chain.” Loz via [The Earth Story]