The area referred to as Grenland is comprised of four communities: Bamble, Porsgrunn, Skien and Siljan.

1500 million years old

The oldest rocks in the area are about 1500 million years old. These Precambrian rocks are found in the western part of the Geopark, as seen in Kragerø and Bamble, west and north of Skien, and in Nome. The landscape was completely different; there were no plants, no large animals as far as we know, and Grenland was part of a continent located south of the equator. These oldest rocks are remnants of processes on this old continent between 1250 and 900 million years ago, when a large mountain chain, the Sveconorwegian orogen, crossed the continent. Rivers flowed, lakes were formed, and there were volcanoes with abundant ashflows and lavaflows. Large granitic melts formed deep down below the earth’s crust. We find all these rocks today as metamorphosed sandstones and schists, rhyolites, gneisses, granites, amphibolites and quartzites.

Erosion and denudation were dominant processes over the next 300 million years, as the mountain chain was modified into a flat landscape. Water flowed in over the old continent, Baltica, and it became flooded by a shallow epicontinental sea. Life flourished in this seawater, and we find remnants from Cambro-Silurian organisms in the typically calceraceous rocks from this period. Corals, trilobites, sea lilies, brachiopods, sponges and cephalopods are among the fossils you may recognise in the rocks of this age. In the Grenland area, the rocks of Cambro-Silurian age are dominated by dark shales interbedded with nodular limestones and thick massive limestones. Some of the limestones are influenced by the later Permian volcanic event, and have been metamorphosed into marble.

Towards the end of the Silurian, a little more than 400 million years ago, the continent of Baltica collided with the large Laurentian continent, and a new large mountain chain was formed west of the Geopark, the Caledonides. The sea covering Baltica withdrew, and left a landscape with beaches, rivers and lakes east of the Caledonides. Late Silurian sandstones from these shores, rivers and lakes can be found in Valleråsen and at Kreppa (Børsesjø, Skien).

For a long time, the geological situation in the Geopark area was quiet, until the late Carboniferous, about 300 million years ago. Another mountain chain rose in the south, and rivers from these Variscan mountains flowed northward, carrying sand and gravel with them. Red sandstones and conglomerates from the late Carboniferous are found in the Geopark, east of the valley Gjerpensdalen and in the eastern part of Porsgunn (Porsgrunn Bymark to Fjelldalen).

In the latest Carboniferous and Permian, the most important event was the creation of the Oslo Rift valley. During a period of about 100 million years, the Oslo area was exposed to extensive rifting and faulting, resulting in earthquakes and volcanism. Lava flows, deep magma chambers and other intrusions are found today as rhombporphyries, larvikites, syenites, pegmatites and maenaitic sills. The large Porsgrunn-Kristiansand fault is one example of the faulting that took place, and the Porsgrunn skaergaard is dominated by deep intrusive rocks.

In the Geopark area, no solid rocks younger than the Permian are found. Geologists assume that the last 250 million years have been a period of erosion and denudation, perhaps with some sedimentation during the Cretaceous period. Whatever the case, every remnant from this long period seems to have been removed by the extensive ice sheets, which covered our country from 2,6 million years ago until 11,500 years ago. There is evidence of about 40 ice ages, and the last one, known as Weichsel, started about 117 000 years ago. The Grenland area was below the ice, and when the ice melted, the landscape was quite different from today. The seawater followed the melting ice, filling up large fiords. The sea bottom was covered with different moraine material, and after a while, sand and clay. A colder period occurred from about 12 650- 12 350 years ago, known as the Ra stage. The ice sheet grew once again, and the well-known Ra morain was formed (Jomfruland, Mølen). A lot of rivers and deltas formed large deposits, which give us important sand deposits today at Eidanger, Nenset (Skien) and Geiteryggen (Skien). The marine clay deposits from this time found along our valleys form very good soils for agriculture.