See:
- http://elhovo.org/en/thtacian-dolmens/
- http://tundja.thracica.com/modules/library/uploads/gderventsmall.pdf
- Georgi Nekhrizov, Dolmens and Rock-Cut Monuments - A Companion to Ancient Thrace, Published Online: 10 APR 2015 - https://www.academia.edu/12100034
- Lyubomir Tsonev, Dimiter Kolev, Megalithic structures and dolmen orientation in Bulgaria Archaeoastronomy and Ancient Technologies 2013, 1(2), pp. 55-84