The granite upland of Dartmoor has attracted settlers coming into the south-west of Britain from the early Bronze Age (Beaker phase) onwards. The long outline of the moor is visible from the South Devon coast, which with its series of good harbours invites intrusion. The navigable lower reaches of the south-flowing rivers—the Plym, Yealm, Erme, Avon, Dart and Teign—and the ridgeways along their watersheds provide convenient thoroughfares across the intervening coastal belt, the South Hams and the hinterland of Torbay, as far as the foot of the main escarpment. Above this steep ascent, where the granite overlies the Devonian rock formations, and where the rivers run in deep wooded gorges, the moorland mass forms a rolling tableland (1000-1400 feet high), broken only by the profile of the numerous upland valleys or by the fantastic eroded outlines of the tors. It is in this zone (light shading, fig. 1) that the evidence for most of the prehistoric settlement and early cultivation is to be found. Between 1400 and 1500 feet, the ground rises markedly again to the uninhabited northern heights (Yestor 2039 feet) and the southern heights (Ryder's Hill 1692 feet), in which the rivers have their sources and which are now blanketed in peat bog (dark shading, fig. 1). The heights apart, Dartmoor contains a broad tract of elevated open country, well-watered grass and heather moor, ideally suited to sustain a primitive pastoral economy. In many places on the marginal slopes and in the upland valleys a sufficient depth of sandy soil has accumulated on top of the rock to make arable cultivation possible; at present, cultivation ceases at about 1000 feet, which is the normal tres line.