1. Alajova-Hrischeva К (1990) Stratigraphy of the lower Eocene sediments in some of the East Bulgarian plateaus. Rev Bulg Geol Soc 60(1):21–31
2. Bakalov P (1921) Upright stones (Dikilitash) Varna District. Nature 22(1):4–6
3. Bakalov P (1922) The stone pillars—Varna district. Bulg Tour 4(3):37–39
4. Bonchev S (1934) The origin (genesis) of the “upright stones” (Dikilitash) or “stick ups” in Varna District. Geol Balk 1(1):5–15
5. Bonchev Е (1949) What we know about the origin of the “upright stones”. Nat Knowl 2(9–10):41–43