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

The Sõru Formation, resting transgressively either on the Lontova Formation or on the crystalline basement, occurs in the northwestern part of mainland Estonia and on the islands of the West-Estonian Archipelago, except Ruhnu (Fig.18).

Rocks of this formation are known only by core sections. The thickness of the formation ranges from 6.2 m at Vihterpalu to 58.2 m at Eikla (Fig.17-40, 178). The Tahkuna drill core (Fig. 17-34) in northern Hiiumaa in the interval of 100.5 to 147 m has been selected as the type section of the Sõru Formation (Resheniya… 1978, Kala et al. 1984a).

The lower part of the formation consists mostly of massive fine-grained quartzose-feldspatic sandstone with thin clay interbeds and films. The upper part is represented by a complex of interbedded argillaceous rocks and sandstones. In both parts the rocks are light-grey with greenish-grey shade, but red and purplish-red patches also occur .