The name Asker Group (Dons & Györy 1967) was established for the sedimentary succession occurring between the basal lava and the underlying peneplain in the central parts of the Oslo Region. Taken in a wide sense, the unit occurs throughout most of the region, but is locally very thin or even missing. The thickest successions known are located in the Skien area to the south (110‑120 m, Segalstad 1976), and along the southern part of the Krokskogen lava plateau in Asker (40‑50 m). Thin successions (<50 m), locally missing, are characteristic in northern and eastern parts of the Oslo Region, in the Skrim area to the west (Rohr‑Torp 1973) and at Holmestrand in Vestfold.
The overlying lava is the basal basalt (B,) except where this is missing, i.e. in the Brumunddal area to the north and in isolated localities elsewhere. The basal flow in B, apparently is progressively older southwards (Ramberg & Larsen, this volume). At Brumunddal, the basal lava is a rhomb porphyry, which overlies 0‑0.3 m of a sedimentary breccia (Rosendahl 1929).
As a rule, the Asker Group is heavily covered by scree from the steep escarpment of the capping lavas. Exceptions are partial outcrops in some crevices and on steep slopes (e.g. at Kolsås) and in a few road‑cuts (e.g. along Billingstadveien and Tanumveien in Bærum, Dronningveien above Sundvollen in the Ringerike area and at Ryggen in the Skien area). Furthermore, resistant benches of sandstone and conglomerate tend to protrude through the scree.
The Asker Group is best known in the Oslo‑Asker area, where it is thickest to the west and comprises 3 formations (Fig. 1); from below: Kolsås, Tanum and Skaugum (Dons & Györy 1967). The only continuous profile through all 3 formations is from the Staverhagen drill‑core, from No. 3 Staverhagen road in Bærum, 0.8 km NW of Billingstad railway station in Asker. The core is described in an unpublished report by Elder and Kanes (1966). All 3 formations appear to be continental deposits. Their principal differences relate to environmental changes which are largely a reflection of increasing volcanic and tectonic activity.