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Gornokredni vulkano-plutonicni strukturi v Juzna Balgarija i tjahnoto otrazenie v relefa (Structures volcaniques et plutoniques du Crétacé supérieur dans le Sud de la Bulgarie, leur impact sur le relief)
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The upper-chalk volcanic and plutonic structures are developed in three regions (Burgas-Strandza, Panagjuriste et Sofija). They are marked by a clearcut central depression and orographic framework surrounding it and belong to the erosional calderas