Calcareous nodules, commonly pigmented by hematite or with hematite rinds, occur especially in its upper part which also contains more sandy material. Anhydrite has been identified in the Staverhagen drill‑core (Elder & Kanes 1966). The core lithologies show a gradational contact between the basal beds and the underlying 1.5 m‑thick breccia of Silurian limestone. The Kolsås Formation is 10‑16 m thick in Bærum and Asker west of Oslo, but appears to be missing east of Oslo. Sections at Kolsås in Bærum are described by Dons & Györy (1967). Two poorly preserved plant fossils of Cordaites and Neuropteris type have been found in the hillside of Kolsås. The formation is also present in the Ringerike area. Brownish‑red, fine‑grained sandstone, etc., with some plant fossils (Cordaites principalis) in the Skien area may belong to the same formation or ` possibly, to an unnamed, local formation.
Lithologically, the Kolsås Formation resembles the continental Rotliegendes of Central Europe (Holtedahl 1953:492). It appears that the Kolsås Formation predates the lava eruptions; according to Elder & Kanes (1966), no volcanic fragments were observed in this formation in the Staverhagen drill‑core. They suggest that the material was derived from a terra rossa type of soil mantling the Cambro‑Silurian bedrock, and interpret the Kolsås Formation to be a fluvial deposit, the environment of which changed from flood plain to braided stream. An arid climate is suggested.