TERTIARY AND RECENT DEPOSITS 409 



smaller streams whose mouths are more or less regularly 

 choked up by sand bars terminate in vleis of various 

 dimensions. The formation of a wide vlei in place of a 

 sharply defined channel is easily understood ; the water 

 flowing into the lagoon cannot escape quickly, but niters 

 slowly through the sand bar ; it therefore stands above 

 the sea level, and owing to its constant movement it 

 laps against the usually soft sandy banks and gradually 

 washes them away, depositing the debris in the deeper 

 portions of the channel. The absence of an open mouth 

 prevents the tide from assisting to keep the channel 

 clear. The mud brought down by the river mingles 

 with the sand blown or washed by rain into the vlei 

 and makes a sandy loam, which tends to form a flat 

 surface somewhat above sea level, so that should the 

 mouth become open for a long period the river will flow 

 through a flat alluvial tract just before entering the sea. 

 Such may be the origin of the flats at the mouths of the 

 Zwart Kops and of the Great and Klein Brak Eivers in 

 Mossel Bay. The same feature is seen at the Kowie 

 mouth, although in this case the channel is maintained 

 by the walls built for the harbour. The Bot and Klein 

 Eivers in Caledon and Bredasdorp have large vleis, 

 which are only open after the winter rains. Many 

 large lagoons, such as Zoetendal and Salt Eiver vleis in 

 Bredasdorp, have quite small rivers flowing into them, 

 and are very rarely open to the sea. Zoetendal vlei is 

 fed by two rivers, and near the mouths of one of them 

 a small stream flows to the sea at certain times by a 

 longer route than would be afforded by the vlei if it 

 were open to the sea near Northumberland Point. In 



