Black layer of Agost Nature · Agost, L'Alacantí, Alicante Image of Agost, not of the place itself. The Capa negra de Agost is an outcrop of the Cretaceous–Paleogene (K/Pg) geological boundary in the municipality of Agost, in the comarca of L'Alacantí, Alicante. It marks the end of the Cretaceous and the start of the Paleogene, an event associated with the extinction of three-quarters of the Earth's species around 66 million years ago. It is one of the most studied sites of this boundary in the world, thanks to its easy access and the wealth of information it provides. It preserves fossils of small marine animals in exceptional condition, from when the area was part of the Tethys Ocean. In Spain there are only two other places where this boundary is visible: Caravaca de la Cruz and Zumaya. Photo: 19Tarrestnom65 (CC BY-SA 4.0) What to see and do in Agost