The upper Oxfordian Arrábida Breccia is a unique lithological type in Portugal and probably in the world because it was formed through several particular geological influences acting simultaneously. The rock is a conglomerate composed of carbonate pebbles of different colours, cemented by a red, ferruginous, clayey carbonate. It is the expression of a fossilized a karst that marks one of the major unconformities of the Lusitanian Basin, which was associated with the early stages of the opening of the North Atlantic. One of the strengths of the candidature of Arrábida for UNESCO’s Mixed World Heritage List is the intimate connection between the natural environment, particularly the landscape (Criterion VII) and the geology and geomorphology (Criterion VIII), and the history and culture of the region and country. The natural occurrences of the Arrábida Breccia (Brecha da Arrábida), the exploration of which ended in 1975 with the creation of the Arrábida Natural Park, are rare and sporadic. The dispersed geographical distribution of outcrops will necessitate the creation of scientific and cultural routes to facilitate visits by the millions of tourists who travel to the Arrábida region each year. The creation of routes in the region is, therefore, one of the objectives of this candidature. In addition, the Brecha da Arrábida conglomerate has been used for ornamental purposes since the late fifteenth century, and was used for the first sculptural and decorative expressions of the so-called Portuguese Late Gothic and Manueline styles, and later during the Baroque because of its colourful varieties. This artistic use of the Brecha da Arrábida, as well as other uses of the material, extends far beyond the actual geographical area of Arrábida. This wider significance provides part of the justification for building a website where users can add information regarding the uses of the Arrábida Breccia in both Portugal and other countries, thereby updating the existing inventory of the material, and constituting a vital component for disseminating knowledge about geology to the public.