The scope of this study is to understand better the pre-Early Ordovician history of the west Pontides of northern Turkey by focusing on the best-exposed part of the Bolu Massif, which is located between Bolu and Yedigoller (Seven Lakes). The Palaeozoic rocks of the west Pontides tectonic belt of northern Turkey comprise a transgressive sedimentary sequence known as 'Palaeozoic of Istanbul.' In a few areas, the basement of the Palaeozoic sequence is exposed, the largest part of which is the Bolu Massif, which is located in the middle of the west Pontides. The lowermost unit of the Palaeozoic of Istanbul in the Bolu area is the Isigandere Formation, which is made up of fluvial red conglomerates and sandstones of Lower Ordovician age. Three different units are exposed unconformably beneath these continental clastics, forming the Bolu Massif. From the structural base to the top, these are as follows: (1) a high-grade metamorphic unit, known as the Sunnice Group); (2) granitoid intrusions, known as the Bolu Granitoid Complex; and (3) a greenschist meta-volcanic sequence (the Casurtepe Formation).