The rock of the Chausey Islands was born 540 million years ago with the Cadomian chain that then culminated at 13,123 ft (4,000 m) and generated granite throughout the Massif Armoricain. Known as the archipelago with the most islands in Europe, Chausey has long been exploited for its blue granite. In the Middle Ages, quarrymen extracted blocks for the construction of Mont-Saint-Michel in the bay. Saint-Malo was also rebuilt thanks to Chausey's stones, and even the sidewalks of London have blue granite borders.