The group's name is based on the municipality of Hole in the Ringerike district; the group comprises the carbonate‑dominated sequences which constitute the upper parts of the marine Silurian succession in the central, western and southern districts of the Oslo Region. These sequences are assigned to our new Braksøya, Malmøya and Steinsfjorden Formations. Representatives of the group are not preserved in the northern districts of the region.
The base of the group is generally coincident with the base of 8c of Kiær (1908). In Ringerike, Modum and Skien this corresponds to the base of our Braksøya Formation and the boundary stratotype for the base of the Hole Group is that defined herein for the Braksøya Formation in Ringerike. The junction with underlying Bærum Group is sharply defined there. In other districts (Oslo, Asker and Holmestrand) the group's base corresponds to the base of our Malmøya Formation, with a gradational lower junction. The composite thickness of the group is 275 m in Ringerike and 240 m in Asker, thinning southwards to 120 m in Holmestrand and 170 m in Skien. The group's top is not preserved in the Oslo district, but its upper boundary to the Ringerike Group in other districts corresponds to the gradational junction between stages 9 and 10 of Kiær (1908).
The Hole Group is correlated with most of the Sheinwoodian and Homerian. Its base is diachronous, younging from Skien and Ringerike to the Oslo district and its top is placed somewhat below the Homerian/Gorstian boundary in the Ringerike district. Several workers have suggested large‑scale regional diachroneity of the junction between the Hole Group and the overlying Ringerike Group (e.g. Spjeldnæs 1967, Bjørlykke 1974 and Ramberg & Spjeldnæs 1978). We still lack precise biostratigraphical documentation of this relationship (see p. 41).