Detailed mapping of the area between Morlaix and St. Michel-en- Greve has been carried out within the northwest part of the Massif Armoricain and three distinct tectono-metamorphic areas have been recognised. In the western area a medium-high grade metamorphic complex comprised of an older basement complex of amphibolites and metasediments, originally Brioverian supracrustals, and Cadomian granitic orthogneisses is recorded These units have been truncated by numerous syn-tectonic and post-tectonic Hercynian granites and this area is termed the St. Pol-de-Leon Metamorphic Complex. In the central part of the area a restricted basin of Devonian-Lower Carboniferous metasediments and basic metavolcanics is recognised. The Morlaix Basin lithologies are of low metamorphic grade and are in contact with the St. Pol-de- Leon Metamorphic Complex across a major tectonic lineament termed the Carantec Shear Belt (CSB). A new stratigraphy of the basinal sequence is presented and the timing of peak Hercynian deformation is established at c. 350 M. y. using available geochronological and field evidence. In the eastern part of the area a low-grade metamorphic terrain composed of Brioverian metabasic volcanics and metasediments occurs, known as the Petit Tregor. This succession is truncated by a multi-component Cadomian intrusive complex which has undergone major, heterogeneous deformation. The recognition of a major shear belt the Locquirec Shear Belt and numerous smaller shear zones within the Moulin de la Rive Orthogneiss Complex has resulted in the re-appraisal of the whole stratigraphy of the Petit Tregor area. Field evidence shows that deformation in the Petit Tregor is somewhat earlier than in the Morlaix Basin and St. Pol-de-Leon Metamorphic Complex and occurred in the period late Cadomian-early Hercynian. Discussion is then given to the timing of regional deformation events in the light of evidence from the study area.