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 Ülgase Formation was referred by Öpik (1929) to the Acrotreta Sandstone and assigned to the local Acrotreta-Lingulella Zone. Subsequently, this part of the succession was defined as the Ülgase Member in the limits of the Pakerort Stage (Müürisepp 1958). In the rank of formation it was first considered by Khazanovich and Missarzhevsky (1982). The formation with a thickness of about 10 m is better fixed in the vicinity of Tallinn and within some 50 km east and south of it. The Ülgase Formation consists of light-coloured very fine- to fine-grained sandstones with interbeds and lenses of greenish-grey clay in the lower part and brownish-grey thin films in the upper part. Its upper boundary is transitional and in the earlier papers the lower part of the overlying Tsitre Formation was regarded as belonging also within this formation (Mens 1984, Popov et al. 1989). The formation contains numerous lingulates of the genera Ungula (including U. inornata), Oepikites, Angulotreta and Cerotreta. The occasional conodonts belong to the genera Phakelodus, Furnishina and Prooneotodus (Kaljo et al. 1986, Mens et al. 1993). Torellella sulcata Missarzhevsky abound. In the argillaceous interlayers the acritarchs, forming the Impluviculus multiangularis - Veryhachium dumontii assemblage are numerous (Volkova 1982, 1990, Volkova & Mens 1988). On the basis of fossil evidence, the Ülgase Formation is assigned to the uppermost part of the Olenus & Agnostus Zone and to the lower part of the Parabolina spinulosa Zone (Table 6).