The relief image of the protected area is roughly determined by anticline ridges and syncline valleys between them. The vibrant activity of endogen and exogenous forces significantly changed the basically simple structure. There are three natural-geographic units according to relief properties: hills, hillocks and valleys. The eastern part of the Posavje Hills is placed into the first unit. The second unit (hillocks) consists of the area between the Sotla and the Bistrica and the area south of the Orlica. There are two major valleys; the Bistrica and the Sotla valley. They run almost parallel until the Bistrica turns east at Trebče.
The surface is divided by valleys, gorges and ravines, owing to diverse geological structure with non-permeable erodible rocks. There are frequent relief inversions. Non-permeable rocks are the reason for the dense river shed. Rivers and streams cut deep valleys with both lateral and down cutting erosion. On the slopes, the weathering suddenly slips and is rinsed away at the same time owing to the different slope and parent material. The outstanding relief is from lithothamnium limestone, resistant to erosion (interesting examples of selective erosion).
Tertiary hills are marked by the above-average presence of lithothamnium limestones that occur mostly in central part. Marl, sand and sandstone prevail on the Sotla's side of the hills. In the lithothamnium limestones belt between Drensko rebro, Zdole, Križan Vrh and Podsreda the surface is karstic, with its typical funnel-shaped holes, caves, abysses and dry valleys. In the Tertiary marl and sandstone belt, there are frequent landslides that occur during the heavy rain periods. The middle part of Sotla hills does not have one central spacious valley but two transversal valleys of the Sotla and the Bistrica.
Here, tectonic rising started later and it hasn't stopped yet. However, it was less intense as in the neighbour Alps. The area is considered prone to earthquakes. In 1974, the area suffered a so-called Kozjansko earthquake with the epicentre in the Zibiški potok valley.
The valley of Bistrica runs along the western part of the area. It widens between Pilštanj and Kozje. The bottom width of the valley changes all the time; it is wider on the flood plains (Lesično, Kozje, Podsreda) and narrower when it passes the resistant rocks of the eastern Posavje Hills. It even becomes a gorge when it cuts through the thick layers of dolomite near Kozje. In Podsreda, the river reaches the layers of poorly-resistant Tertiary rocks and it makes a sudden turn to east. Between the Orlica and the Bukovica, near Trebče, there is a 3 km epigenetic gorge.
A wide plain between the Bistrica and the Sotla opens at Zagaj and continues to Kumrovec plain in Croatia.
Hills and hillocks were cut from NW to SE by the Sotla that cuts through the east Orlica below Kunšperk (and after Zelenjak gorge) and then turns south. Flood plains along the Sotla are much wider than the ones along the Bistrica, owing to the less resistant Tertiary sediments. Hills, hillocks and plains belong to the fluvial or fluvial-Karst relief type. Surface and subterranean Karst phenomena are located within the so-called »isolated Karst area« that belongs both to Tertiary and Mesozoic.
Various forms of erosion such as accumulation, weathering, denudation and karstification formed the followin geomorphological phenomena in Kozjanski Park: