I.Introduction This paper describes the geological structure of a strip of country lying between four and 12 miles east of St. David's, and occupying the central portion of the small peninsula of north-western Pembrokeshire. Although in a district made classical by the researches of many famous geologists, this tract has received but passing notice, and, up to the present, has remained practically unknown geologically. The area is about 30 square miles in extent, measuring rather more than seven miles from east to west and four miles from north to south. It is bounded on the east by the G.W.R. main line to Fishguard, where the line runs along the Western Cleddau valley, between Welsh Hook and Mathry Road (Heathfield). On the north it adjoins the Abereiddy-Abercastle area (Cox, 1915, p. 273) and the Pen Caer area (Cox, 1930, p. 274), while to the south is the Brawdy-Hayscastle area (Thomas & Jones, 1912, p. 374). The St. David's area (Green, 1908, p. 363) and the Trefgarn area (Thomas & Cox, 1924, p. 520) lie respectively a few miles to the west and south-east. The country here described is represented on the Ordnance Survey six-inch quarter-sheets, Pembrokeshire 15 N.E., S.E., N.W., S.W., 16 N.W., S.W., 8 S.E., and 9 S.W., and it comprises a portion of the area embraced by the Geological Survey one-inch maps, Sheet 40 (Old Series) and Sheet 209 (New Series). The district forms part of the Pembrokeshire 400-foot peneplain, and shows the monotonous topography typical of the in