The smaller fragments of rock that break off are carried by the wind, and sometimes water, and they in turn erode other rocks. Gradually they become so small that they are merely grains. The same wind that helps to form sand particles also shapes the landscape in other ways. Given the right conditions, the easily transported grains of sand will accumulate to form sand dunes. These extraordinary features can vary from small heap-like structures of only about a metre in height to enormous sand mountains of 1,000 metres.
Some dunes often reach several kilometres in length.
They are often found in large groups known as sand seas or ergs. The largest of these, covering around 560,000 square kilometres, is in the Arabian Desert. An important and intriguing characteristic of sand dunes is their tendency to move around. Typical annual movement of sand dunes is between 10 and 20 metres, depending upon size, but in extreme cases small dunes may travel up to 50 metres in a year. But sand is just one of a number of landscape types to be found in the desert.