Complete question:
Which of the following statements about sand dunes is true?
Choose one:
a. They are made of sandstone, a type of sedimentary rock.
b. In cross-sectional view, they show cross beds of sand.
c. Sand dunes that are covered with vegetation often erode more quickly than do non-vegetated dunes.
d. They can migrate downwind more than 2 km in a year.
Answer:
b. In cross-sectional view, they show cross beds of sand.
Explanation:
Winds can transport many different-sized particles of material from the ground through the process of erosion. These particles might travel many kilometers until they deposit somewhere. Regions with no vegetation have too much dust and grain-sized particles that move and crush each other, increasing erosion. This process is known as eolic erosion. Dunes are winds deposits, formations that are typical of deserts. Dunes are orientated according to the direction and speed of winds and the presence of loose material and vegetation. Strong winds transport sand until it collides with an object or obstacle, where it is deposited, originating a new dune.
Dunes are very dynamic formations that are constantly, being remodeled by the direction and strength of the winds. Dunes always have a soft slope and a sharp one. The soft slope faces the direction where the wind is coming from and is the product of the wind erosion on the same dune. The opposite sharp slope is protected from the wind and is the deposition place, where the wind losses strength and the carrying material falls, depositing in this side of the dune. Dunes are arranged as cross-bedding forms, reflecting the erosion and deposition pattern.
This is how erosion and deposition work together to form a dune.
