They are built up by Precambrian metamorphic rocks .
Geologically , the Stob Earth Pyramids are part of the Blagoevgrad Graben that consists of lakebed , alluvial and sloping deposits dated from the Pliocene and Pleistocene epochs .
Stob Earth Pyramids are built up by Precambrian metamorphic rocks .
Geologically , the Stob Earth Pyramids are part of the Blagoevgrad Graben that consists of lakebed , alluvial and sloping deposits dated from the Pliocene and Pleistocene epochs .