The distribution of Mesolithic sites in Wales is controlled to a great extent by the terrain, for physiographically, Wales is a highland block defined on three sides by the sea and for the greater part of the fourth side by a sharp break of slope. Geologically the Principality is composed almost entirely of Palaeozoic rocks, of which the 600-foot contour encloses more than three quarters of the total area. There are extensive regions above 1,500 feet and 2,000 feet and in the north the peaks of Snowdonia and Cader Idris rise to 3,560 feet and 2,929 feet respectively. Indeed North Wales consists of an inhospitable highland massif, skirted by a lowland plateau and cut deeply by river valleys, providing only limited areas for settlement. The hills and mountains of Snowdonia with their extension at lower altitudes into the Lleyn Peninsula, and the ranges of Moelwyn, Manod Mawr, Arenig Fach and Cader Idris, are discouraging obstacles to penetration, save for a short distance along the river valleys. To the east of these peaks are extensive tracts of upland plateau dissected by rivers, bounded on the west by the vale of the river Conway and cleft by the Vale of Clwyd. To the east of this valley lies the Clwydian Range and further again to the east these uplands descend with milder contours to the Cheshire and Shropshire plains.To the south the district merges into the uplands of Central Wales, which are continuous until they are replaced by the lowland belt of South Wales.
VIII.—The Millstone Grit of the North-Wales Border