CÓRNAGO: Fossil of the Mesozoic Era. Rock formed by 95% due to the accumulation of pressed shells. Clam shells very crushed and deformed that were deposited, forming large shell, along the length of a delta plain as was La Rioja (Spain) in the Secondary or Mesozoic Era (250 to 65 million years). Very extensive next to the corrals of Juancaliente (18-3-2017, José Javier Latasa).