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The Skaugum Formation, named after Skaugum in Asker, is characterized by a predominance of grains of volcanic rocks. It consists of sandstones and conglomerates, generally with a reddish‑brown calcareous matrix coloured by hematite (Elder & Kanes 1966). Low‑angle cross‑bedding and scour surfaces are common. Green shale from the NE side of Semsvatn shows a bentonitic composition and may represent tuff layers (unpublished report by E. Roaldset & J. A. Dons). A silicified fragment of a tree trunk (Dadoxylon sp.) was found close to the base of the formation at Tanum (Høeg 1937b). In the Oslo‑Asker area the Skaugum Formation is 15 m thick above Semsvatn in Asker and decreases rapidly in thickness northeastwards. Thus, only a few patches of similar lithology overlie the Tanum, Formation in Kolsås (Dons & Györy 1967). The Skaugum Formation is also known in the southernmost part of the western hillside of the Krokskogen lava plateau, and from the Skien area. Similar lithologies are common in sedimentary intercalations between the lavas.

The Skaugum Formation’s lithology reflects strong volcanic activity in the source area. According to Elder and Kanes (1966), the Skaugum Formation in the Staverhagen drill‑core is a fluvial deposit, suggesting a change in environment from flood plain to braided stream.