 The plains are interspersed by horst and graben structures, believed to have broken the Pannonian Sea's surface as islands.The tallest among such landforms are Ivanšćica and Medvednica north of Zagreb—both are also at least partially in Hrvatsko Zagorje—as well as Psunj and Papuk that are the tallest among the Slavonian mountains surrounding Požega.Psunj, Papuk and adjacent Krndija consist mostly of Paleozoic rocks from 300–350 Mya.