Palaeozoic era
The oldest rocks are approximately 300 million years old and from the time when the area was coved by the sea. These are the layers of slate clay, silicate sandstone and conglomerate. No fossils were found in them.
 
Mesozoic era
The area surrounded by the Sava, the Savinja and the Sotla is marked by various and numerous rocks from Triassic period. The most noticeable parts of relief, such as the Vetrnik, the Orlica and the Rudnica were built of these rocks. Brown sandstone, sandstone with oolite limestone inserts, dolomite and marl were formed in early Triassic period. A wealth of sea fauna (snails, foraminifera) was found in these layers. The age of rocks and past living conditions were revealed on the basis of this fauna. Beside dolomites, dolomite marls and limestones that were formed in late Triassic period, there are also volcanic diabase and volcanic ash that prove the volcanic activity of that period. Several waterfalls and streams took shape in Triassic layers. The Bistrica, a typical river of medium high mountains, carved most of its riverbed into Triassic rocks. It cut through the western part of the Orlica anticline, and the space between Trebče and Zagaj became very narrow. The picturesque gorge is one of the best preserved river gorges in the eastern Slovenia. Jurassic-Cretaceous layers consist of limestones, colourful marl limestones, slate claystones and marls with the abundance of sea micro flora and fauna.

Cenozoic era
Tertiary sediments of Pannonian Sea filled the area among the Vetrnik, the Orlica and the Rudnica.  They cover a significant part of Kozjanski Park and form interesting geological profiles and boundaries, Karst caves, abysses, springs, funnel-shaped holes, tuff waterfalls and the abundance of sea organisms.

The layers developed as marls, sandstones, lithothamnium limestones and marl limestones. There are precious findings of sea shells, urchins, mosses and other micro organisms in Tertiary layers. A particularly well-preserved fish, probably from Lates family, was found in the layers near Olimje.

An extinct shark's tooth was found in middle Miocene layers on the southern slope of Orlica. The tooth is 10 cm wide and 10.5 cm long. A bit less famous but very interesting are the petrified red algae that form a crusty cover which later becomes an egg-shaped bulb – a rhodoid. If they get stuck together, they form a rock called rhodolite.

There are some minor Karst caves and abysses in Tertiary layers of Dekmanca, Križan Vrh and Podsreda. Quaternary sediments are represented in the form of diluvium, slope rubble and alluvium. Diluvial sediments are badly or not rounded fragments. Slope rubble consists of light grey dolomite. Alluvial deposits consist of fine pebbles, sand, clay stone and clay.

Tectonics
The surface of Kozjanski Park is very folded and divided into smaller clods by numerous tectonic faults. From north to south there are many anticline and syncline structures. The fold axes are oriented mostly form east to west. The sequence of anticlines and synclines from north to south: Rudnica anticline, Laško syncline, Litija anticline, Senovo syncline, Orlica anticline

There was volcanic activity in Triassic and Tertiary period. Numerous thermal springs, hydrothermal ore formation and recent earthquake (1974) remind us on vibrant tectonic activity in the area.

The occurrence of minerals
Minerals discovered in Kozjanski Park tell the tale of vibrant geological history. Mineral phenomena that can be seen in Kozjanski Park: bauxite in Trebče over the Bistrica gorge, Tertiary brown coal location east of Kozje, aragonite formations in the abandoned iron mine on the Rudnica, pyrite crystals found in Olimje and numerous thermal springs.