The fourth letter of the Greek alphabet was named after delta. Its sign was a triangle - a three-sided figure. Today the word delta means the triangular group of islands at the mouth of a river.

Deltas are formed by the rivers themselves. Large rivers may carry sand and fine soil as they sweep towards their mouths. Near their outlets, rivers usually slow, and can less easily carry their burdens. Sand and silt are dropped when rivers enter a large - body of water.

Not all rivers form deltas. Oceans currents may be strong enough to sweep the river water away before sand and silt are deposited. Or the deposits may themselves be swept away.

The fertile soil of the Nile delta has made it famous. Man’s history in this delta dates back for thousands of years.