The group's name is based on the municipality of Bærum. The group comprises the mixed clastic and carbonate marine sequences found throughout the Oslo Region which we assign to the Solvik, Sælabonn, Rytteråker, Vik, Ek, Bruflat and Skinnerbukta Formations; all of these units are defined and described below.
The base of the Bærum Group throughout the region is everywhere coincident with the lowermost occurrence of rocks earlier assigned to stage 6. In the Oslo, Asker and Holmestrand districts this corresponds to the base of our Solvik Formation; the boundary stratotype for the base of the Bærum Group is therefore that defined for the Solvik Group on Hovedøya in the Oslo district. In other districts the group's base corresponds to the base of our Sælabonn Formation. The base in these latter districts is clearly disconformable; this is most easily seen in the northernmost districts where basal units infill and overlie karstic surfaces in the Caradoc (to early Ashgill) Mjøsa Limestone.
The composite thickness of the group is approx. 400 m in the Bærum district (Fig. 4). The group is thickest (600‑650 m) in the northern districts of the Oslo Region but its top has not yet been definitely identified there. In all other districts the top of the group coincides with the boundary between 8b and 8c as defined by Kiær (1908). As discussed below (p. 32), under our description of the overlying Hole Group, this junction is sharply defined in the Ringerike and Skien districts, but is gradational in Oslo and Asker.
The group appears to range in age from the early Llandovery to the mid Sheinwoodian (riccartonensis or low rigidus Biozone). The group's top is diachronous and youngs from the Ringerike and Skien districts towards Asker and Oslo.