Derivation of name. The name alludes to Ringstranda on the eastern shore of Lake Mjøsa, where the lower part of the new formation is exposed in a couple of thrust-slices (Vogt 1924, fig. 13; see also Skjeseth 1963, pl. 1). The exposures at Vinjulandet, Ringstranda, Steinsodden and Evjevika along the eastern shore of Mjøsa south of Moelv are described by Brøgger (1882a), Kiær (1917), Vogt (1924), Strand (1929) and Skjeseth (1963).
Stratotype. The Lower Allochthon section at Steinsodden, Mjøsa District (Skjeseth 1963, fig. 16) is designated as the basal stratotype (see Redalen Member).
Members. The Ringstrand Formation comprises the Brennsætersag, Redalen, Tømten, Evjevik and Sky-berg members (Fig. 8).
Lithology. The formation is dominated by mudstone but sand- and siltstones also occur, particularly in the lower part. Two thin limestone intercalations are present near the top of the formation. For lithological description, see in particular Vogt (1924) and Skjeseth (1963).
Boundaries. In the Autochthon the Ringstrand Formation rests unconformably on weathered crystalline basement and a basal conglomerate is always developed. In the Parautochthon and Lower Allochthon the formation rests unconformably on the Vangsås Formation. The lower boundary is also here associated with a conglomerate and is readily identified by the shift from the monotonous quartzitic Ring-saker Member to the finer-grained and more heterogeneous Ringstrand Formation. The formation is upwards bounded by the Hawke Bay unconformity and is everywhere unconformably overlain by the Alum Shale Formation.
Distribution and thickness. The Ringstrand Formation is developed in the Autochthon, Parautochthon and Lower Allochthon of the Mjøsa District. It is 2 m thick in the Autochthon at Risbekken (Høyberget, new measurement, see also Strand 1929) and 14 m in the Parautochthon at Brennsætersaga (Høyberget, new measurement, see also Vidal & Nystuen 1991). The formation is presumably about 50-60 m in the Lower Allochthon around Lake Mjøsa, but due to tectonic disturbance it is impossible to state an exact thickness, and it may theoretically even be up to about 80-100 m.
Age. Late Early Cambrian. The formation represents the upper main part of the Heliosphaeridium dissimilare-Skiagia ciliosa acritarch Zone and possibly ranges into the Volkovia dentifera-Liepaina plana Zone (see treatment of the individual members for references). It straddles the Holmia kjerulfi and 'Ornamentaspis' linnarssoni trilobite zones (see entries on the Tømten and Evjevik members).
Remarks. Several thin lithostratigraphic units, referred to as formations of the Holmia Series in older literature (Skjeseth 1963 and subsequent authors), are present between the Vangsås and the Alum Shale formations. They are here combined into one formation for which the name Ringstrand Formation is proposed. This unit is more comparable in scope with the under- and overlying formations, although still thinner. The various units described by Skjeseth (1963), some of which were renamed by Vidal & Nystuen (1991), are in a modified or recombined form ranked as members of the Ringstrand Formation (see separate sections below). The thin Lower Cambrian autochthonous deposits present in the Mjøsa District (Strand 1929, Skjeseth 1963, Bjørlykke 1979) are also assigned to the Ringstrand Formation.
From A.T. Nielsen & N.H. Schovsbo, 2007