问答题Delta
 The large river best known to the ancient Greeks was the Nile of Egypt. They spoke of the river with admiration and called Egypt “the gift of the Nile”. The reason for this was, first, that the Nile brought water to a rainless desert and, second, that once a year, the river overflowed its banks, leaving, as the water went back, a new layer of fertile soil.
 The flood waters carry in them soil (called silt) from the upper parts of the river valley to the lower parts, and so to the sea. But as the river meets the sea, the sea acts as a barrier and forces the river to drop the silt it is carrying.
 There are no tides in the Mediterranean to carry the silt away, so year after year it collects at the mouth of the Nile, and the river must find its way around islands of silt to the always more distant Mediterranean. In this way, a vast area of fertile soil has been built up at the mouth of the Nile and out into the sea. The river water splits up to form small branches winding across the area. To the ancient Greeks, the mouth of the Nile looked like the drawing.
 Now we sometimes name things after the letters of the alphabet they resemble: a U-turn, an I-beam a T-square, an S-bend, and so on. The Greeks did the same. The triangular area of land built up at the mouth of the Nile looked like the fourth letter of the Greek alphabet delta (△) and so this was the name they gave it. The word is now used for all areas of land formed at the mouth of rivers which flow into tideless seas, even when they are nor triangular in shape. The Mississippi delta, for example, is not shaped at all like the Greek delta, as you will see if you look at a map.