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.
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