The ferruginous sandstone used to build the walls of Guédelon castle was extracted from the site's quarry. It forms a sandstone lens exploited in the open in the 19th and 20th centuries for construction. There remains today the western edge of the lens that offers a diversity of rock quality. From bottom to top, there is a bench of cold stones, a bench of firm stone, and blocks of semi-firm to soft stone mixed with sandy soil, all used on the Guédelon construction site. The base of the quarry is made up of a very dense and homogeneous compact mass of cold, bluish rock of the kind we see here.