The results of those processes are large plains in eastern Slavonia, Baranya and Syrmia, as well as in river valleys. The plains are interspersed by horst and graben structures, which are believed to have broken the Pannonian Sea surface as islands. Psunj, Papuk and Krndija consist mostly of Paleozoic rocks that are 350 - 300 million years old. Požeška Gora and Dilj consist of much more recent Neogene rocks, but Požeška Gora also contains Upper Cretaceous sediments and igneous rocks forming the main ridge of the hill and representing the largest igneous landform in Croatia. A smaller igneous landform is also present on Papuk, near Voćin. The two mountains are possible remnants of a volcanic arc related to Alpine orogeny—uplifting of the Dinaric Alps.