Sand Dunes

1    Sandy deserts contain enormous volumes of sand eroded from mountains and carried to the deserts by wind or water. The huge quantities of sand that make up sandy deserts are blow about into dunes of various shapes.

2    Ridge dunes form where there are large amounts of sand, generally in the interiors of deserts, and winds blow in one direction. Under there conditions, parallel ridges of sand, known as transverse dunes, form at right angles to the wind.

3    When the direction of the wind changes so that it comes from different directions, star-shaped dunes form from the massive amounts of sand in desert interiors. Star-shaped dunes are relatively stable dunes that reach incredible heights, up to 80 meters high in some deserts, and are quite common in massive deserts such as the Sahara.

4    Crescent dunes form on the edges of deserts where there is less sand and where the winds blow mainly in one direction. These dunes, which are also known as barchan dunes are less stable than star-shaped dunes and can shift as much as 20 meters per year as winds blow over the outer curves of the crescent in the direction of the pointed ends.

Answer Choices (choose 4 to complete the chart):

(1) Ridge and crescent dunes form where the winds blow from one direction.

(2) Crescent dunes are also known as barchan dunes.

(3) Star-shaped dunes form where the winds blow from different directions.

(4) Transverse dunes are created parallel to the wind.

(5) Ridge and star dunes form where there is a lot of sand.

(6) Star-shaped dunes are more stable than crescent dunes.

(7) Crescent dunes form where there is less sand.

參考答案：

第一欄：（5）、（7）

第二欄：（1）、（3）