Existing stratification, dynamics, and other characteristics of the relief are closely related to the petrographic composition and geological settings. Mesozoic and Tertiary layers are abundantly present in Kozara Mountain, although Kozara basically is made up of older rocks (Paleozoic, Precambrian). Mesozoic rocks are prevalent in the southeastern and northwestern part of Kozara Mountain.
The position of old sediments, primarily of Paleozoic and volcanic rocks, indicates that the uplift of Kozara was accompanied by volcanic outbursts.
In the park, we find sediments that dominate both in the structure of Kozare. These are primarily Eocene flysch and Mesozoic sediment rocks. There is, next to them in the southern part of the zone, lower-ocean limestone.
In geological terms, the Kozara National Park is a quite diverse place. The central and northern parts are composed of layers of Tertiary (Eocene) flysch, presented in very different forms. The peaks in the zone above 800 m, and even much lower, are generally made of limestone. Cretaceous flysch is common in the region from the eastern periphery of Benkovac to the northern edge of Gola Mountain. It consists of marly limestone, shale and sandstone. Most of the basin of the Starenica River, Bijela kosa, Hrastova i Jurišna kosa and Razboj are at gabro-dolerite soil. Limestone and dolomite are abundant in the area of Kozarački kamen, Sokolina, Vrnovica, Vrnovička glava, Zečji kamen and Jankovica, and in the valley of Starenica, at Gumline and Gola Mountain, near Jarčevica and Pavkovci and southern from Hrastova kosa. Complexes of sandstones and shales are to be seen in the south area. Small blocks of serpentines are in the southeastern part of the park, south of the stream Jovovača.
The representation of individual stem substrate in the National Park is as follows: flysch 70.22% 20.52% gabrodolerit, sandstones and shales 4.19%, dense limestones and dolomites of 3.84%, 1.23% serpentinite.