Jolkičev, Nikola. 2006. “The Cretaceous/Paleogene (K/Pg) Boundary in the Mezdra and Lyutidol Syncline, Vratza District (West-Fore Balkan, Bulgaria)”. Geološki Anali Balkanskoga Poluostrva 67 (1), 41-49. https://doi.org/10.2298/GABP0667041J.