The results of those processes are large plains in eastern Slavonia, Baranya and Syrmia, as well as in river valleys, especially along the Sava, Drava and Kupa. The plains are interspersed by the horst and graben structures, believed to have broken the Pannonian Sea surface as islands. The tallest among such landforms in Slavonia are Psunj, and Papuk—flanking the Požega Valley from the west and the north. These two and Krndija, adjacent to Papuk, consist mostly of Paleozoic rocks which are 350 – 300 million years old. Požeška Gora and Dilj, to the east of Psunj and enveloping the valley from the south, 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, as well as Moslavačka gora, west of Pakrac, are possible remnants of a volcanic arc related to Alpine orogeny—uplifting of the Dinaric Alps. The Đakovo – Vukovar loess plain, extending eastward from Dilj and representing the watershed between the Vuka and Bosut rivers, gradually rises to the Fruška Gora south of Ilok.