The influence of the tide in bringing back the finer particles gives the sea near the mouth of the Zambesi a clean and sandy bottom.

This process has been going on for ages, and as the delta has enlarged eastwards, the river has always kept a channel for itself behind.

Wherever we see an island all sand, or with only one layer of mud in it, we know it is one of recent formation, and that it may be swept away at any time by a flood; while those islands which are all of mud are the more ancient, having in fact existed ever since the time when the ebbing and flowing tides originally formed them as parts of the delta.

This mud resists the action of the river wonderfully.

It is a kind of clay on which the eroding power of water has little effect.