The Ancient Erg of Hausaland, and Similar Formations on the South Side of the Sahara

northern Nigeria is formed into low rounded ridges, and that these are ancient dunes bearing witness to the former prevalence of desert conditions. Their outlines have been modified by erosion, and they are well covered by woodland in many areas so that it is difficult for an observer on the ground to appreciate their arrangement. From the air, however, the dune patterns are clear, and they are strikingly apparent on blocks of air photographs that have been re-photographed to give laydowns on scales of about i : 50,000 and 1 : 125,000 (Grove 1957). With the permission of the Directorate of Overseas Surveys I have used these print laydowns, and prints of the original photographs on a scale of about 1 : 30,000, to construct a map of the dead erg of northern Hausaland (Fig. 1). This erg is described, the nature of similar formations in the same latitudinal belt of Africa is summarized, and their significance is indicated in the conclusion. The ancient erg of Hausaland in north-east Nigeria The dominating feature of the erg is the mass of long dune ridges running mainly in a direction ENE.-WSW., but swinging round to run NE.-SW. near the northern frontier. The dunes appear on the photographs as pale-coloured strips, some a few hundreds of metres, others a few kilometres wide, running across country for tens of kilometres. Between the ridges are darker, narrower stripes, with pools of water lying here and there. The photographs were taken in October and November, in the early dry season, and before the onset of the dust-laden Harmattan from the north-east. These dunes are of a similar kind to the linear dunes of the Libyan desert, which are still active and run in the direction of the dune-forming winds, i.e. winds stronger than about 20 k.p.h. The mechanism of their formation is not understood, but Bagnold (x953) has suggested the possibility that sand might be swept into long parallel ridges as a result of secondary circulations in the air; thermal instability giving rise to horizontal roller vortices. The live dunes of Libya and the Sahara, as Sandford has noted (1953), are aligned NNW.-SSE. and N.-S. to the north of the tropic, and veer round to run ENE.-WSW. further south. This change of direction is no doubt related to westerly deflection of the north winds, and the arcuate form of the linear dunes in the ancient erg of Hausa? land may well represent the curved track of north-easterly winds in the arid period of the past when the dunes were shaped. But the situation is complicated by the fact that south-west winds are prevalent during the rainy season of the present day, and winds from that quarter might also have had some influence on dune-building in the arid past. Further study of the surface wind velocities in northern Nigeria and the area to the north is needed in this connection. The arrangement of other dune forms in the Hausaland erg shows that the problem of relating dune and wind directions is not a simple one. A belt of country about 60 kilometres wide on the south side of the longitudinal dunes shows a less regular pattern of rather bare ridges, their crests about 200 to 300 metres apart, and with trees and bushes in the intervening hollows. In general they run north and south, but the pattern is distorted by valleys, normally dry, forming the local drainage system. The ridges tend to run down the valley slopes, across the contours. A similar set of transverse ridges and hollows occupies a small area on the north-western side of the