Grenlands geology

Different Origins

We find rocks of various origins: igneous rocks that were once molten, sedimentary rocks that were once loose materials like sand, clay, and gravel, and metamorphic rocks that have undergone changes because the physical conditions changed.

1,500 Million Years

The oldest rocks in Grenland are 1,500 million years old. We do not know exactly where Grenland was located at that time; perhaps we were far south of the equator. We know that water and wind eroded mountains and rocks on an ancient continent, loose materials were deposited, and eventually compressed into sandstones and mudstones.

Volcanic Activity

At the same time, there was volcanic activity, and lava flowed over older rocks. Ash and other volcanic products were deposited together with the lavas, and large areas of melt solidified at depth to form granites. The rocks from this time are called basement rocks, and they are found on the western side of the Grenland area, for example in Bamble, at Rognstranda, in Kilebygda, Skien Vestmark, and Luksefjell. During the period from 1,250 to 900 million years ago, Grenland was part of a large mountain range; the so-called Sveconorwegian mountain range.

Formation of Continents

Mountain ranges are formed when continents collide, and this is not a gentle process: large packages of different rock layers are folded, pushed down to great depths and over each other, and many cracks and faults develop. Such movements result in frequent earthquakes.

The basement rocks in Bamble and Skien ended up deep within this mountain range, and the old sandstones and mudstones became quartzite and mica gneiss, while the granites, lavas, and ash layers became granitic gneisses and amphibolites. The old mountain range is now worn down, and therefore the rocks that were once deep below the surface have now come up to the surface. This explains why we can see and walk on these beautiful gneisses, for example at Rognstranda today.

Erosion of the Mountain Range

Over the next 300 million years, this mountain range was also eroded. About 575 million years ago, there was volcanic activity in the area once again. The center of this volcano was within the borders of today’s Nome municipality, and it was a very unusual volcanism, traces of which we can still find today. The volcano gave rise to what we call the Fen Complex.

Flooding

The continent that Grenland was on, Baltica, was flooded by seawater during the period from around 550 to 400 million years ago. The landscape was worn down to sea level, and water flowed in from the northwest. In the shallow sea that formed, life was abundant. We find traces of this life in rocks in Porsgrunn and Skien today, as fossils in limestone and shale that were deposited in the sea.

Rivers ran over the lowland, carrying sand, gravel, and clay. When the river water met the sea, sand and gravel settled on the seabed and became sandstone and conglomerate. The sea level varied a lot during the three periods covering this time, the Cambrian, Ordovician, and Silurian. This meant that the sediments deposited varied between sand, lime, and clay.

Formation of Limestone

Many of the animals that lived in the sea had calcareous shells. This was a fairly new and successful “invention,” and many different animal groups rapidly evolved. There were forms such as trilobites, brachiopods, cephalopods, corals, crinoids, and graptolites. Many of these animal groups are now extinct, while descendants of others still live today.

When the animals died, their shells sank to the bottom, and after thousands of years, the calcareous shells became limestone. The rocks from this period are called Cambro-Silurian rocks, and these are found as clearly layered rocks in the outcrops at Høgenhei, along Frierfjorden, and in a broad belt along the Ælva and north toward Luksefjell. At the Bjørntvedt quarry, limestone from this period is extracted, and at Kjørholt and Dalen mines in Brevik, Ordovician limestone is still quarried for cement production. At Kapitelberget, there are remains of a beautiful Silurian coral reef.

Collisions

Toward the end of the Silurian, Baltica collided once again with another continent, and a large mountain range developed in the sea to the west. The sea that covered “Grenland” gradually retreated southeastward. The upper part of the Silurian is characterized by river and freshwater sandstones. These can be found at Valleråsen and at Kreppa near Børsesjø. For a long period, “Grenland” may have been dry land; in any case, we do not find sediments or other rocks in the area until the very end of the Carboniferous, a little over 300 million years ago.

Storm Winds

At that time, yet another new mountain range had risen south of Baltica. From this mountain range, many rivers flowed, and together with storm winds, they transported gravel and sand over the lower-lying areas to the north. This can be found as red sandstones and conglomerates in areas east of Gjerpensdalen, from Porsgrunn Bymark to Fjelldalen.

The last part of the Carboniferous and the Permian was a special period in Grenland’s geological history. Again, there was volcanic activity, this time connected to the rifting of the Oslo Rift. Once again, lavas flowed over the landscape. In today’s geological landscape, we can find basalts, for example east of Gjerpensdalen. Not all the melt reached the Earth's surface.

Rocks

Rocks that solidify deep in the crust are called intrusive rocks. The bedrock in the Porsgrunn archipelago, in Langangen, Vealøs, the hills in Siljan, and other areas consists of such intrusive rocks from the Permian period, larvikites and syenites. The volcanic activity lasted from about 300 to 250 million years ago.

It is in the area between Porsgrunn and Brunlanes that we find the largest deposits of Norway’s national rock, larvikite. Within Porsgrunn’s borders, in Tvedalen, there are a couple of quarries where larvikite with the sought-after, beautiful blue shimmer is extracted.

Larvikite

Most larvikite quarries are located in Larvik municipality. After the Permian period, we have not found any solid bedrock in Grenland. The landscape has since been shaped and carved out by many ice ages, and it is also during the ice ages that our soil was formed. But that is another story..