These are limestones, cemented together from shells og organisms that lived in the sea about 400 million years ago.
Bamble is dominated by two types of landscape with completely different geological histories. In the west the landscape is dominated by ancient basement bedrock, whereas limestones are characteristic further east from the peninsula of Langesundhalvøya. The division between these two rock types is clearly visible in the terrain and is easy to see from Rognstranda.
The lovely, ice-polished bedrock on Rognstranda is mainly ancient gneisses (ca. 1.3 billion years old), and they once lay deep within a mountain chain. From the smooth beach bedrock a sharp edge juts up in the east, with “horizontal stripes”. These are limestones that are “only” about 450 million years old. At that time, the continental plate that Norway was a part of lay south of the equator, and the entire country was covered by a shallow tropical sea teeming with life. Remains of the shells from this life make up the limestones that one sees for example in the small cliff on the other side of Rognstranda. This edge also dominates the landscape along the E18 highway through Bamble, for example at Høgenhei.
Locally these edges have their own name: “flauer” (= smooth, rounded rock face). The limestones that we see in these can also be seen many other places in Bamble, for example, in the bay of Steinvika and on Langesundstangen.