LANCASTER: A REPLY 283

FORMATION OF THE HOLOCENE LAKE CHILWA SAND BAR

AREPLY

N. Lancaster, Walvis Bay

The comments of Shaw and Crossley on my paper "Formation of the Holocene Lake Chilwa sand bar" illustrate some of the problems which may be encountered in trying to date lake levels, especially when erosional features are produced.

The occurrence of Iron Age material within the 631 m beach (Crossley, personal com- munication) would appear to provide excellent evidence for a relatively recent age for this shoreline. However, I would interpret this evidence as reflecting the reworking, in early Iron Age times, of sands which were laid down much earlier, when the sand bar was originally for- med in the late Pleistocene or early Holocene.

In my interpretation of the age of the sand bar, I tried to offer the simplest explanation which was consistent with the evidence available to me. I agree that caution is necessary when interpreting the 14C ages of calcareous deposits from the vicinity of Chilwa Island, but I am inclined to feel that the contribution of local runoffto the chemistry of waters in a shallow lake which is well mixed by wave action may not be significant. My field work in 1977 sugges- ted to me that the shallow water clays, from which the ~4C dates were obtained, did pass shorewards into sandy clays, which in turn passed laterally into the sands of the 631 m beach. As a result of this, I felt that the correlations between the ages of the clays offshore and the 631 m beach were reasonable.

I still feel that the Chilwa sand bar was formed in the late Pleistocene or early Holocene in the manner outlined in my paper, but agree with Shaw and Crossley that my interpretation of the subsequent history of the lake, on which I did little work, is oversimplified. I look forward to seeing the results of these investigations. I also suspect that detailed studies of the later Holocene deposits of many African lakes would yield similarly valuable reinterpre- tations of lake level fluctuations.

Address of author: N. Lancaster, Desert Ecol. Res. Unit, Namib Desert Res. Station P.O. Box 953, Walvis Bay 9190, South West Africa