The Pannonian Basin took shape through Miocenian thinning and subsidence of crust structures formed during the Late Paleozoic Variscan orogeny. Paleozoic and Mesozoic structures are visible in Papuk and other Slavonian mountains. The processes also led to the formation of a stratovolcanic chain in the basin 12–17 Mya; intensified subsidence was observed until 5 Mya as well as flood basalts at about 7.5 Mya. The contemporary tectonic uplift of the Carpathian Mountains cut off the flow of water to the Black Sea, and the Pannonian Sea formed in the basin. Sediments were transported to the basin from the uplifting Carpathian and Dinaric mountains, with particularly deep fluvial sediments being deposited in the Pleistocene epoch during the formation of the Transdanubian Mountains. Ultimately up to of sediment was deposited in the basin, and the sea eventually drained through the Iron Gate gorge. The result is large plains, particularly in river valleys, and especially along the Sava, Drava, and Kupa rivers. The plains are interspersed with horst and graben structures, believed to have broken the Pannonian Sea's surface as islands. The tallest among these landforms are Ivanšćica and Medvednica, north of Zagreb. Parts of Moslavačka gora, along with igneous landforms on Papuk and Požeška gora mountains in Slavonia to the east, are possibly remnants of a volcanic arc from the same tectonic plate collision that caused the Dinaric Alps.