Fossils in the Sigri Park are found in two visible layers of volcanic rock.
The upper layer (or horizon), approximately 20 meters thick, is a pyroclastic flow with large stones, mainly irregularly shaped lava fragments. Nine fossilized trunks were uncovered in this horizon during the excavations, with circumferences varying from 1.20 m to 3.20m. These fossilized trunks are characterized by intense brown-red colours on the inside and white–green colours on the outer surfaces, and the interior wood structure is also clearly visible. Secondary layers of silica minerals are frequently found on the outer surface of the trunks or in small cracks which originated from the dissolution of the initial petrified material.
The lower horizon consists of fine-grained volcanic material, mainly volcanic ash. Five petrified coniferous gymnosperm trunks were found in their initial location within this horizon. The systematic excavation fully revealed the root systems of these trunks, proving beyond doubt the indigenous nature of the Petrified Forest. These fossilized trunks are mainly characterized by brown and red colours. The fine-grained volcanic material contributed to the preservation of fragile parts of the plants like small branches, leaves from angiosperm trees, specifically various types from the Lauraceae family such as the leaves of the cinnamon(Cinnamomum polymorphum), seeds, and parts of root systems. In this horizon, a grassy layer full of branches, needles, and pine cones (Pinus sp.) was also discovered.
