The geology of the Blue Mountains Inlier is complex and incompletely understood due to the rugged terrain and difficulties of access. Previous studies have attempted to correlate units across the whole of the inlier, but this conflicts with geochemical studies. We have undertaken a geological study in the northern Blue Mountains in and around the small rural town of Chelsea in Portland, Jamaica. The geology is recorded as a late Cretaceous and Paleogene succession, which extends into the communities of Swift River, Fruitful Vale, Shrewsbury and Shirley Castle. The Cretaceous rocks consist of a sequence of volcaniclastics (purple andesitic tuffs and conglomerates) and andesites (interpreted as either lava flows or dykes) belonging to the Spanish River Formation, which is assigned a late Campanian age. This sequence has been intruded by granitoid stocks and andesitic dykes. At least three phases of granitoid intrusion are recognised, with the earlier phase altered by the intrusion of the later phases. The earliest granitoid intrusion is stock-like, has an extensive hornfelsed contact, and the primary fabric has been strongly hydrothermally altered with feldspars replaced by a fine-grained intergrowth of kaolinite and quartz. The second phase is a larger, stock-like intrusion, situated to the south of the first phase, and consists of unaltered granodiorite with pokilitopic quartz. The third phase consists of dyke-like bodies of granitoid that intrude the earliest phase and its hornfelsed halo. The earliest phase has yielded radiometric dates indicating an early Maastrichtian age, but the relative ages of the other phases have yet to be determined. The granitoids are associated with at least three phases of mineralisation (magnetite-pyrite skarns in the hornfels, early barite-specularite-calcite-chalcopyrite veins, and later barite-chalcopyrite-calcite veins). The Cretaceous rocks are overlain unconformably by a Paleogene succession. This consists of the grey to dark grey grainstones and packstones interbedded with grey shales of the Chepstowe Formation (mid-to-upper Palaeocene), overlain by shales and sandstones of the Richmond Formation (mid-to-lower Eocene). The Richmond Formation is overlain unconformably by rocks belonging to the White Limestone Group of Middle Eocene to Miocene age. The Paleogene succession shows no indications of mineralisation. The structure revealed by mapping of the early Paleogene rocks indicates a series of N-S folds and faults, with some of the river courses following synclines.