Limits of the study area which encompasses Yenipazar district of Bilecik province extend to the Gölpazarı district of Bilecik province in the west and the Göynük district of Bolu province in the east. The Paleogene sequence exposed in the area begins with Paleocene age terrestrial sediments (Kızılçay Group). Eocene age marine units and overlying sediments have a lithology starting with yellow colored, coarse, firm sandstones and continuing with brown colored, loose sandstone levels (Güvenç Formation). There are a lot of Nummulite fossils in this sandstone level. Stratigraphically, the Nummulite-bearing level is overlain by the sandstone containing Rotularia spirulaea  (Lamarck) and Ampullina cf. vulcani (Brongniart) species that lived in the Late Eocene shallow sea. This sandstone level ends with the mudstone and marl lithology (Güvenç Formation), which is in the central parts of the study area is defined by a lot of Turritella sp.-bearing samples. A boundary relationship of angular unconformity between this unit and a younger continental conglomerate, sandstone and mudstone unit is clearly visible in the central parts of the study area. Specimens such as Polymesoda convexa (Brogniart), Gari protracta Mayer and Globularia gibberosa santistephani (Grateloup) are identified from the samples taken from the sandstone-mudstones in the Kürnüç valley in the easternmost part of the study area (Kürnüç village, Göynük district, Bolu province,). Thus, the sandstone-mudstone unit in this valley is described as a new level representing the brackish water, lagoon, shallow marine conditions during the Egerian (Late Oligocene) time, previously unknown in the study area.

Key words: Egerian, Late Oligocene, Kürnüç Village Valley, Gastropoda-Bivalvia, Stratigraphy