Precambrian time

The geology in the Gea Norvegica Geopark tells a story that covers over 1500 million years. The oldest bedrocks in Norway are called basement rocks, and large areas with ancient basement are found in Kragerø, Bamble, Porsgrunn and Skien. Here, the bedrock has undergone several long episodes of folding and faulting, resulting in high mountain chains. These mountain chains are now eroded away by wind and weather. The bedrock that today comprises the smooth, polished coastal islands in Kragerø, Bamble and Porsgrunn once lay far down under the earth's crust in the roots of these high mountains. Rock types were formed such as gneisses, granites, amphibolites and quartzites. In several places, these rock types lay the foundation for commercial working of i.e., gravel, quartz, iron ore and various elements and minerals such as nickel, lead, zinc, copper and apatite.

Throughout the long history of our country, the land's appearance has changed continuously, and the bit of the continental plate Baltica, which Norway is a part of, has a dramatic and varied history. Baltica once lay south of the equator, and has steadily moved north, as part of larger and smaller land masses. About 580 million years ago an extraordinary volcanic activity occurred on Baltica. At that time the Fen-volcano was active, and we find remnants of this volcano in the community of Nome. The Fen Formation is world famous for its volcanic limestone deposits.

Paleozoic time

Cambrian, Ordovician and Silurian

About 540 million years ago, Baltica was a part of a severely eroded-down flat landscape, seawater flooded over the ancient basement deposits, and the tropical sea that covered the whole of South Norway was teeming with life. Primitive squids and trilobites swam in the water and brachiopods, corals, sea lilies and bryozoans lived on the bottom. Many of the animals had calcareous shells, and these shells accumulated in thick layers of calcareous mud, mixed with layers of sand and clay. These were later compressed into layers of limestone, sandstone and shale. These layers are found today as landmarks of Rognsflauene, the steep escarpment along Frierfjorden and on the eastern side between Porsgrunn and Skien. In Porsgrunn and Brevik the limestone from the ancient seafloor of the Ordovician Period is commercially worked.

Volcanism and earthquakes in Permian time

The different mountain chains that formed during the Silurian – Devonian and Carboniferous Periods were the result of different large continental plate collisions. In the beginning of Permian time, this led to almost all of the continental plates being united in one great super continent, Pangea. This kind of constellation is not stable in the long run, and this super-continent eventually began to split up. About 300 million years ago our area began to break up, and an extensive system of faults developed. This was the beginning of the creation of the Oslo Rift, a region that stretches from Langesundsfjord in the south to north of Lake Mjøsa. Geologists from all over the world know of the Oslo Rift, and the special geology that is found here. The long faults developed into volcanoes, both fissure volcanoes and subsequently big, lava cone volcanoes. Lava flowed out of the volcanoes and solidified into basalts and rombeporphyries. Deep down in the ground huge masses of syenites and larvikites and in some places the coarse-grained rock type pegmatite were formed.
The volcanic activity was not gentle, it was a disturbed time with many earthquakes and tremors. The area outside of the rift was also affected, molten material flowed through a system of fractures and solidified in a variety of different intrusions. Many rare minerals were formed as a result of the heating effect within these fissures. Among geologists, Langesundsfjord is known for its great variety of minerals formed in pegmatites and other places in connection with volcanism in the Permian. Some of these minerals were found for the first time in the world here, for example, thorite, the source for thorium.

Ice ages in Quaternary time.

For unknown reasons, the fracturing of the Oslo Rift ceased. What is now the land of Norway went into a geologically calm period. Within the area of the Gea Norvegica Geopark we have no evidence of further geological activity until the large Ice Age started 2.6 million years ago. Then, the climate deteriorated dramatically, and glaciation has more or less covered Norway for perhaps as many as 40 ice ages, interrupted by warmer interglacials. The one we know best is the last Ice Age, the Wechselian, that started 117,000 years ago, and, not least, the period when this ice sheet melted. Today's landscape is shaped by the glaciers, and the soil we cultivate and live on is mainly formed after the last Ice Age. The smooth polished rock faces, sand and silt deposits, the Lågen river bed and the clays along the water courses of Skien, up toward Siljan and in Lågendalen valley, are all remnants of the large ice sheets' activity and retreat. Not least, the Ra, the big end moraine that surrounds the whole of Scandinavia, is a distinctive monument from the last Ice Age in our area. The Ra, together with Jomfruland and Mølen, is a result of a colder period after the ice sheet had begun to melt.
12,800 years ago, it became colder again, and the ice sheet grew and then stood still for about 300 years, from about 12,650 until 12,350 years ago. The land was pressed down by the weight of the ice, and in front of the ice sheet was the sea. The ice sat on the sea floor and large glaciers calved into the Arctic Ocean. Stones, sand, silt and clay were pushed up in front of the ice edge, transported by the ice and the river water flowing beneath the glacier. As the climate became milder, the ice sheet receded, and the large ridge of loose sediment deposits was left behind. As the weight of the ice gradually lessened, the land rose, first quickly and then more slowly, in a process that continues today. The long ridge of loose deposits eventually reached sea level, became sorted by the waves, and resulted in the beautiful cobblestone beaches we see today on Jomfruland and on Mølen.

The geological history is long and exciting. There is every reason to learn more about it and to protect the evidence of these different worlds from the past that can be seen in the Geopark.