Fine ash blown by the wind from the Oldonyo Lengai’s eruptions has collected into rolling black dunes that spread across the Salei and serengeti plains.
One of the dunes, called Shifting Sands, is easy to reach from Olduvai Gorge. From the visitor centre, cross the gorge, taking the track that passes the guides houses on the opposite (north) rim. The track passes to the left of a hill (Soit Naibor), and about 2km further on you will see an isolated black sand dune to your left.
Shifting Sands is a fine example of a barchans or crescent shaped dune. It is about 9m high and 100m along its curve. The constant wind blowing from the east pushes sand grains from the back of the dune up its firm, rippled, windward slope. The grains scramble upwards only to topple over the steep leeward slope. Thus the dune marches westward, grain by grain.
It is easier for many grains to roll sideways rather than uphill, so they form the tapered horns of the crescent, pointing downwind. But the wind speeding round the curves pushes the sand around the corner, where it joins the march forward as a unit thus keeping the dune together. Look for the dated concrete markers, east of the dune; these give an indication of the progress. Since 1969 it has covered about 17 metres per year; in another thousands year it may have reached the main road.
