South of Porsgrunn, across Frierfjorden and further into Bamble by Herre, there is an extensive fault, the results of a period of earthquakes about 300 million years ago.
This fault is clearly visible where the river "Ælva" run into a fault zone, and the city of Porsgrunn grew up along the river's banks. If one looks further to the south, one sees that it sparkles from the many smaller water bodies that lie in the depression that the fault formed in the landscape. We can follow this depression all the way to Kristiansand.
In the Langesundsfjord-Eidanger area, there are many rare minerals. Many of these were first discovered and described here, and the occurrence are known among geologist in many parts of the world. One of these mineral is thorite, discovered in the 1928 at Løvøya. Thorite contains the until then unknown element thorium, which was named after the Norwegian god Thor.
As in Skien, many of the deposits in Porsgrunn are limestone's that were formed at the bottom of a tropical sea about 450 million years ago. This limestone, which is full of fossils, is selected for industrial use for example by Norcem in Brevik. The limestone from Porsgrunn is used in cement, but generally it can be used for much else, for example, in plastics, paper and toothpaste.