The range of relief within the 20 km square is 174 m (Fig. 2, on folding sheet). The highest parts of the plateau lie close to the western edge of the map, the lowest in the north-east corner. The regional slope of the plateau is thus from the Xingu basin east and north across the present watershed into the Araguaia basin. Wells dug below the plateau at the Red Fazenda revealed that the sandstones were weathered to at least a depth of 30 m; a nodular laterite layer no more than 1 m thick was encountered between 11 and 12 meters below the surface, and water level was 29*1 m at the end of the dry season in 1968. In Figure 3 the form of the plateau surface is indicated by generalized contours, drawn by eye in Base Camp, Fig. 3a; and trend surface analyses carried out later on a computer, Figs. 3b and c. The data employed to produce these maps were the summit and ridge top altitudes, as indicated by closed form lines and spot heights on the 1 150,000 map. The linear trend surface, Fig. 3b, has a gradient of 5*6 m per km in a direction 083 ? true and accounts for 73*5% ofthe variations in altitude on the plateau surface. The cubic surface, Fig. 3c, gives a better fit and accounts for 81% of the altitudinal variations. The latter surface comprises three elements. In the centre is an easterly sloping element with a gradient of 8 m per km separating two other elements with gentler slopes. That in the west has a slope of 1*4 m per km in a southerly direction, contrary to the slope of the Xingu headwaters which drain towards the north, while in the east there is a surface with a northerly slope of between 2 and 5 m per km, in conformity with the headwaters of tributaries of the Rio das Mortes (a tributary of the Araguaia). The present-day water parting between the Xingu and Araguaia basins follows a somewhat devious north to south course through the 20 km square. A stream line contour map, Fig. 3d, another type of generalized contour map, drawn by eye only using the altitudes of selected points along the long profiles of the rivers as shown on the 1 -.50,000 map, indicates how asymetric this divide is. The gradients of the headstreams of the Araguaia are up to twice as steep as those of the Xingu. The divide bulges westward in three localities where the dendritic patterns of the steeply sloping headstreams of the three major valley systems draining east to the Araguaia, Ao, Ai and A2, have their sources (Fig. 4, on folding sheet). Complementarily, the divide bulges east at Airstrip Campo and White Sands valley. The density of streams and dry valley axes east of the divide is 6-i km/sq. km, namely more than twice as high as the density in the southern part of the area west of the divide where it is 3-8 km/sq. km and approximately five times greater than the density in the northern part of the area tributary to the Xingu. Presumably these differences reflect to some extent the greater aggressiveness of the Araguaia head? waters but the similarity in stream pattern on either side of the divide in the southwestern quadrant of the 20 km square and the intermediate value of the stream density suggest possibly that within the 20 km square the same rock type underlies the whole of the Araguaia basin and the southern one-third of the Xingu basin. In the southern and north-central areas there is a prominent scarp wrapping around the sources of the easterly draining streams. Its crest is rarely coincident with the major divide and may be up to 2 km east of it. The area between the scarp and the height of land along the divide is occupied by a gently sloping convex slope. What is believed to be the same scarp is traceable in a less continuous fashion around the head of the valley system, Ai, within which Base Camp was located. Exposures in the face