Depositional Landforms in Arid Geomorphology
Now to study the depositional landforms carved by Arid geographies let’s again imagine that you are standing in a vast desert, and the wind is blowing fiercely, carrying fine grains of sand with it. But what happens when the wind slows down or suddenly dies? The sand grains, which were once lifted and carried far away, now begin to settle on the ground. Over time, this continuous deposition of sand gives birth to stunning depositional landforms, uniquely shaped by the wind.
The most remarkable depositional landforms in deserts are sand dunes — majestic, wavy hills of sand that look like the sea frozen in time. Let’s explore their fascinating types one by one.
1. Sand Dunes
💭 What are Sand Dunes?
Sand dunes are mounds or hills of loose sand formed by the deposition of wind-borne sand particles. They are commonly found in hot and dry deserts where vast open spaces and abundant loose sand are available. However, for a dune to start forming, there must be an obstacle (like a rock, vegetation, or any solid object) that acts as a barrier, causing sand to accumulate behind it.
Now, depending on the wind’s behavior and the availability of sand, sand dunes can take different shapes. Let’s explore them.
2. Barchans: Crescent-Shaped Dunes
💭 Visualize This:
Imagine standing in the Thar Desert of Rajasthan. You notice several sand dunes shaped like a crescent moon (C-shape) with their wings or horns pointing away from the wind. These are called Barchans.
👉 Why do they form?
- When wind consistently blows from one direction, and there is plenty of loose sand, Barchans start to form.
- The sand accumulates behind an obstacle and slowly builds up into a crescent-shaped dune.
- Over time, the wind pushes the sand forward, making the dune migrate in the wind’s direction.
- The wings of the crescent always point away from the wind, helping you determine the wind’s direction.
👉 Key Features of Barchans:
- Crescent-shaped.
- Wings point away from the wind.
- Form in areas with constant wind direction and abundant sand.
- Common in deserts like Sahara, Thar, and Arabian Deserts.
3. Parabolic Dunes: The Reverse Barchans
💭 Visualize This:
Now, imagine you enter a desert area where some vegetation (like small shrubs or grass) is present. Interestingly, you now spot dunes that look similar to Barchans, but the horns (wings) point towards the wind — the opposite of Barchans. These are called Parabolic Dunes.
👉 Why do they form?
- In areas where sand is partially covered by vegetation, the plants hold the sand in place.
- As the wind blows, the sand around the vegetation is blown away, creating a dune where the arms are anchored by vegetation and the central part advances with the wind.
- This is why Parabolic Dunes are considered as “Reverse Barchans.”
👉 Key Features of Parabolic Dunes:
- Arms point towards the wind (opposite of Barchans).
- Form in areas with partial vegetation and loose sand.
- The wind direction remains the same as in Barchans, but the dune looks reversed.
- Common along desert margins, coastal dunes, and semi-arid areas.
4. Seif Dunes: The One-Armed Dune
💭 Visualize This:
Imagine you spot a dune that looks like a Barchan, but it has only one arm or wing — the other wing seems to have disappeared. This unique dune is called a Seif Dune (Seif means sword in Arabic).
👉 Why does it form?
- Sometimes, wind direction slightly shifts over time, causing one arm of the Barchan to erode, leaving behind only one prominent wing.
- The remaining wing stretches in the direction of the wind, making it look like a long sword-shaped dune.
👉 Key Features of Seif Dunes:
- Only one arm or wing present.
- Forms when wind direction slightly shifts over time.
- Common in Middle Eastern and African deserts.
5. Longitudinal Dunes: Long Parallel Ridges of Sand
💭 Visualize This:
As you continue your journey, you notice long, parallel sand ridges stretching endlessly into the horizon. These ridges are low in height but very long, running parallel to the wind direction. These are called Longitudinal Dunes.
👉 Why do they form?
- When the supply of sand is limited, but the wind blows consistently from one direction, the sand accumulates in long, narrow ridges.
- These ridges may stretch for several kilometers but are relatively low in height.
- Between two longitudinal dunes, you’ll find sand-free, bare surfaces called Reg or Hammada.
👉 Key Features of Longitudinal Dunes:
- Parallel ridges of sand.
- Form when sand supply is poor but wind is consistent.
- The spaces between the dunes are called Reg or Hammada.
- Seen in vast desert regions like Sahara, Arabian Desert, and Kalahari Desert.
6. Transverse Dunes: Sand Ridges Perpendicular to the Wind
💭 Visualize This:
Now, imagine a sand dune that stretches like a long wall across the desert, cutting perpendicularly to the wind direction. This is called a Transverse Dune.
👉 Why do they form?
- When the source of sand is abundant (like an old river bed or coastal region) and the wind blows consistently, the sand accumulates in the form of long ridges at right angles (90°) to the wind direction.
- Over time, these dunes look like a series of long, wavy sand ridges aligned perpendicular to the wind.
👉 Key Features of Transverse Dunes:
- Aligned perpendicular to the wind.
- Require abundant sand supply and constant wind direction.
- Often found in areas with old river beds, coastal deserts, or sand-rich plains.
- Common in the Sahara Desert, Kalahari Desert, and Arabian Desert.
✅ Quick Recap of Dune Types
Sample Question
Q. Transverse dunes are formed perpendicular to the prevailing wind direction, despite the intuitive expectation that sand should move along the wind’s path. Discuss the geomorphological processes responsible for this phenomenon. (150 words)
Answer: Transverse dunes are large, wave-like ridges of sand that form perpendicular to the prevailing wind direction. Their formation can be attributed to several geomorphological processes:
- Abundant Sand Supply: Unlike longitudinal dunes, transverse dunes require a continuous, extensive sand source, often from an elongated feature (e.g., a beach or riverbed) positioned at right angles to the wind.
- Deposition Due to Wind Turbulence: As the wind blows across the sand-rich surface, small ridges begin to form. These ridges create localized wind resistance, slowing down airflow and causing more sand to settle, reinforcing the perpendicular alignment.
- Dune Migration: While the dunes are aligned perpendicular, individual sand grains still move in the wind’s direction through saltation and creep. This results in gradual forward migration of the entire dune.
Thus, transverse dunes form due to a combination of wind consistency, sand abundance, and depositional dynamics, making their perpendicular alignment a natural outcome of aeolian processes.
