Mens, K. & Pirrus, E. (1997). Cambrian. In: Raukas, A., Teedumäe, A. (eds). 1997. Geology and Mineral Resources of Estonia. Estonian Academy Publishers, Tallinn. 436 pp. ISBN 9985-50-185-3. 

The Ruhnu Formation is distributed in the southwestern part of Estonia, while its range in the middle of southern Estonia is plausible. It lies transgressively on the Lower Cambrian and is overlain by the Upper Cambrian or Lower Ordovician rocks.

The Ruhnu drill core in the interval of 706.8 to 748 m (Fig. 22-257) has been selected as the type section for the Ruhnu Formation (Kala et al. 1984a). The maximum thickness of the formation (41.2 m) has been established in this borehole and it decreases towards the north.

The Ruhnu Formation is represented by light well‑sorted, fine‑ to very fine‑grained quartzose sandstones with only a few thin interbeds of dark‑grey, in the lower part sometimes variegated, argillaceous rocks. The basal part is often marked by a layer of very coarse‑grained sandstone containing over 10% of quartz with gravel or pebble grain-size, or with both. The top of the formation is usually cemented by carbonates and pyrite. Often an admixture of feldspars and micas (muscovite) occurs. Glauconite is lacking. Heavy mineral assemblage is characterized by the prevalence of transparent minerals dominated by zircon and tourmaline. Among opaque minerals, detrital leucoxene and ilmenite occur in almost equal quantities or the former prevails slightly. Clay minerals are dominated by illite, the content of kaolinite often reaches 30‑35%. Chlorite is rare reaching occasionally 10% in the lower part of the formation.