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PARALLEL RIDGES
Aeolian Depositional Landforms
Stoss vs. Slip-Face Physics · Barchans, Parabolic & Seif Dunes · Longitudinal vs. Transverse · Loess & Thar Dhrian (28 Micro-Realms)
The Five NCERT Sand Dune Geometries & Loess Blankets (NCERT Chapter 7)
Wind is a supreme sorting agent. While fine dust travels high in suspension to blanket distant regions as Loess, saltating sand grains accumulate behind obstacles to form Sand Dunes with a gentle windward slope and a $34^\circ$ leeward slip-face. Depending on wind constancy, sand supply, and vegetation, NCERT Figure 7.16 classifies five primary dune forms: 1) crescentic Barchans (wings point downwind), 2) vegetation-pinned Parabolic Dunes (reversed barchans), 3) one-winged Seifs, 4) parallel Longitudinal Dunes (poor sand supply), and 5) perpendicular Transverse Dunes (abundant sand supply).
I. Dune Aerodynamics, Stoss vs. Slip-Face Physics & Migration (NCERT)
Obstacle Prerequisite
- Dry hot deserts are good for sand dunes
- Obstacles to initiate dune formation
- Are equally important (NCERT first rule)
Stoss vs. Leeward Slip-Face
- Windward (Stoss) slope: Gentle ($10^\circ\text{–}15^\circ$)
- Sand saltates up windward ramp to crest
- Leeward (Slip-face): Steep ($32^\circ\text{–}34^\circ$)
34° Angle of Repose
AVALANCHE
- Maximum stable slope of dry loose sand
- When crest oversteepens beyond $34^\circ$
- Sand avalanches down the slip-face
Dune Migration & Cross-Beds
- Erosion on stoss + deposition on lee
- Dune marches downwind (up to 30 m/yr)
- Preserves inclined Foreset Cross-bedding
Aeolian Sand Ripples
- Smallest aeolian bedform (cm-scale)
- Wavelength matches mean saltation hop
- Coarsest grains concentrate at ripple crests
Nebkhas (Coppice Dunes)
- Embryonic sand mound trapped around
- Desert scrub bushes (Calligonum/Capparis)
- Streamlined wind-shadow sand tongue
Draa (Mega-Dunes) & Gassi
- Draa: Colossal compound erg ridges (100–400 m)
- Gassi: Rocky sand-free interdune corridor
- Used as caravan highways across Sahara
II. Barchans, Parabolic Dunes & Seifs (NCERT Fig 7.16 Core)
1. Barchans (Crescentic)
- Crescent-shaped dunes (NCERT Fig 7.16a)
- Points or wings directed AWAY from wind
- i.e., Horns point DOWNWIND!
Barchan Conditions (NCERT)
- Form where wind direction is constant
- And moderate throughout the year
- Over uniform surfaces with limited sand
Barchanoid Ridges
- As sand supply increases, barchans crowd
- Lateral horns fuse into wavy scalloped rows
- Transitional stage to Transverse dunes
2. Parabolic Dunes (NCERT)
- Form when sandy surfaces are partially
- Covered with vegetation (NCERT Fig 7.16b)
- Parabolic dunes are REVERSED BARCHANS!
Barchan vs. Parabolic
- Both have the SAME wind direction
- Barchan: Center lags, horns race downwind
- Parabolic: Vegetation pins horns upwind!
Hairpin Blowout Dunes
- Central blowout nose advances far downwind
- Leaves long parallel vegetated arms
- Characteristic of semi-arid Thar margins
3. Seif Dunes (One-Winged)
- Arabic "Saif" = Curved Sword
- Similar to barchan with a small difference:
- Seif has ONLY ONE WING or point (NCERT)!
III. Longitudinal, Transverse & Star Dunes (NCERT Fig 7.16 Core)
Seif Scale & Wind Shift
- Occurs when there is a shift in wind
- Lone wing of seif grows very long & high
- Reaches 100 m height & 100 km length
4. Longitudinal Dunes
- Long ridges of considerable length
- But low in height; PARALLEL to wind
- Supply of sand is POOR & wind is constant!
5. Transverse Dunes
- Aligned PERPENDICULAR ($90^\circ$) to wind
- Form when wind direction is constant
- And source of sand is ABUNDANT (NCERT)!
Longitudinal vs. Transverse
∥ · POOR SAND
⊥ · RICH SAND
- #1 UPSC Prelims comparative trap
- Longitudinal: Parallel + Poor sand supply
- Transverse: Perpendicular + Abundant sand
Star Dunes (Rhourds)
- Multi-directional seasonal wind regime
- 3 or more radiating arms from central peak
- Remain stationary & grow 300+ m tall
Sand Sheets & Whalebacks
- Flat to gently undulating sand plains
- Zibar: Coarse bimodal sand without slip-faces
- Selima Sand Sheet (Eastern Sahara)
Echo, Climbing & Falling Dunes
- Topographically anchored obstacle dunes
- Echo/Climbing dune on windward cliff face
- Falling dune cascades down leeward scarp
IV. Loess Silt Blankets & Indian Thar Desert Depositional Landforms
Loess (Wind-Blown Silt)
- Fine dust ($0.02\text{–}0.05\text{ mm}$) carried in suspension
- Deposited outside desert margins
- Homogeneous, unstratified, friable & fertile
Desert vs. Glacial Loess
GOBI → CHINA
OUTWASH SILT
- 1) Desert source: Gobi deflation $\to$ N. China
- 2) Glacial source: Outwash rock-flour silt
- Blown across North America & Europe
Global Loess Provinces
- Loess Plateau (China): 100–300 m thick; Yellow River (Hwang Ho)
- Vertical cleavage enables Yaodong cave homes
- Ukraine Chernozem & Argentine Pampas
Thar "Dhrian" & "Dhoros"
& DHORO DEPRESSIONS
- Dhrian: Shifting sand dunes in Marusthali
- Dhoro: Long trough between longitudinal dunes
- Sam, Khuri (Jaisalmer) & Churu barchans
Thar Parabolic Dominance
- Because Thar is a vegetated semi-arid desert
- Anchored Parabolic & coalesced raked dunes
- Cover the largest area in Western Rajasthan!
Bhur, Karewa Loess & Miliolite
- Bhur: Wind-blown sand ridges in Upper Ganga-Ramganga plain
- Kashmir Karewas & Son-Belan loess caps
- Saurashtra Miliolite: Aeolianite limestone
Master NCERT Fig 7.16 Key
- Completes Aeolian Landforms triad (50–52)
- All 5 NCERT Fig 7.16 dune types mastered
- Transitions to Realm 53 (Glaciers)
Core Distinctions
• Barchans: Crescent-shaped dunes with two wings/horns directed downwind (away from wind); form on bare uniform surfaces with constant wind.
• Parabolic Dunes: Form when sandy surfaces are partially covered with vegetation; they are reversed barchans (horns anchored upwind, convex nose pointing downwind) with the same wind direction.
• Seif Dunes: Similar to barchans except a Seif has only ONE wing or point, created when there is a shift in wind conditions.
• Longitudinal Dunes: Long ridges of low height aligned parallel to wind; form when wind is constant and supply of sand is POOR.
• Transverse Dunes: Long wavy ridges aligned perpendicular to wind; form when wind is constant and the source of sand is ABUNDANT.
Sand Dunes are stratified, cross-bedded mounds of medium quartz sand ($0.1\text{–}0.5\text{ mm}$) moved locally by saltation. Loess is an unstratified, porous, calcareous blanket of fine yellow silt ($<0.05\text{ mm}$) carried hundreds of kilometres in suspension.
NCERT Fig 7.16 Dune Matrix
Orientation, Wings & Sand Supply Key