Introduction:
Sand dunes are fascinating natural formations found in deserts, coastal areas, and even on some planets like Mars. They are created by wind and are composed of loose sand particles. Sand dunes are constantly changing due to the movement of sand and wind. In this context, we will discuss the statement that is not true about sand dunes.

Explanation:
Temporary Hills of Sand:
Sand dunes are indeed temporary hills of sand. They are constantly being shaped and reshaped by the wind. As the wind blows, it lifts and carries sand particles, which then settle and accumulate, forming dunes. Over time, the shape and size of sand dunes can change dramatically.

Formed Around a Barrier:
Sand dunes are often formed around a barrier, such as rocks, vegetation, or buildings. These barriers obstruct the wind flow, causing sand particles to accumulate and form dunes. The presence of a barrier creates a wind shadow, where sand particles settle and accumulate.

Cannot Be Used as Landmark:
Sand dunes can be used as landmarks in the desert. Due to their distinct shapes and sizes, sand dunes can help guide travelers and explorers. They provide visual cues and can be used as reference points for navigation. However, it is important to note that sand dunes may shift and change over time, so relying solely on them as landmarks may not be entirely reliable.

Permanently Remain at One Place:
The statement that is not true about sand dunes is that they permanently remain at one place. Sand dunes are not static; they are constantly moving and changing shape. The movement of sand dunes is driven by wind, and they can shift, migrate, and even disappear over time. The process of sand movement is known as aeolian transport, where sand particles are lifted and carried by the wind, causing dunes to migrate.

Conclusion:
Sand dunes are dynamic natural formations that continuously change due to wind and sand movement. They are not permanent structures and can shift and migrate over time. However, sand dunes can still serve as landmarks in the desert, providing visual cues and reference points for navigation.