Coastal Erosional Landforms
Wave-Cut Cliffs & Terraces · Sea Caves, Arches, Stacks & Stumps · Blowholes & Geos · Indian Exemplars (28 Micro-Realms)
The Destruction of Rocky Headlands & Straightening of Shores (NCERT Chapter 7)
Almost all sea cliffs are steep and may range from a few metres to 30 metres or more. At the foot of Wave-Cut Cliffs, lashing waves and rock fragments excavate basal notches and hollow out Sea Caves. As caves enlarge and coalesce across a headland, they form a Sea Arch; when the cave/arch roof collapses, isolated resistant rock masses remain standing off the coast as Sea Stacks, which eventually plane down into Stumps—leaving a flat Wave-Cut Terrace above an offshore Wave-Built Terrace and reducing an irregular coast into a smooth, straight shoreline.
I. Wave-Cut Cliffs, Basal Notches & Marine Terraces (NCERT Core)
Wave-Cut Sea Cliffs (NCERT)
- Almost all sea cliffs are steep
- Range from a few m to 30 m or more
- Upper summit may be flat or tilted
Basal Wave-Cut Notch
- Surf & pebble corrasion focus at high-water
- Undercuts a horizontal groove (Notch)
- Unsupported overhanging rock collapses
Landward Cliff Recession
- Repeated notch cutting & rockfall
- Forces sea cliff to retreat inland
- Widens the rocky shore platform
Wave-Cut Terraces (NCERT)
- Flat or gently sloping rock platforms
- At the foot of sea cliffs
- At elevation above average wave height
Wave-Built Terraces (NCERT)
- Rock debris fallen from receding cliff
- Gets ground & carried offshore by backwash
- Deposits a Wave-Built Terrace in front!
Strata Dip & Cliff Angle
- Seaward-dipping beds: Prone to rockslides
- Landward-dipping beds: Stable vertical face
- Controls coastal cliff slope stability
Raised Beaches & Dead Cliffs
- Tectonic uplift or sea-level drop
- Strands former wave-cut terrace high & dry
- Leaves abandoned inland "Dead Cliff"
II. Headland Dissection: Sea Caves, Arches, Stacks & Stumps (NCERT Core)
Headlands & Bays
- Discordant coast with alternating rocks
- Soft shale/clay erodes into recessed Bays
- Hard basalt/granite juts out as Headlands
Headland Wave Refraction
- Shoaling bends wave crests around headland
- Concentrates destructive assault on sides
- Accelerates cave & arch perforation
Sea Caves (NCERT)
- Lashing waves against base of cliff +
- Rock debris hurled at joints/faults
- Creates hollows that widen into Sea Caves
Sea Arches / Natural Bridges
- Two caves on opposite sides of a headland
- Cut backward & unite into a tunnel
- Leaves rock roof spanning a Sea Arch
Sea Stacks (NCERT)
- Roofs of caves & arches collapse
- Resistant rock masses remain isolated
- Standing as Sea Stacks off the cliff!
Stumps & Skerries
- Sea stacks are temporary features (NCERT)
- Basal wave undercutting topples the stack
- Leaves low sub-tidal rock Stump / Reef
5-Stage Headland Chain
- Classic coastal evolutionary sequence
- Joint $\to$ Cave $\to$ Arch $\to$ Stack $\to$ Stump
- Twelve Apostles (Victoria) & Old Harry Rocks
III. Pneumatic Blowholes (Gloups), Geos, Coves & Platform Pitting
Blowholes / Gloups
- Surge compresses air at back of sea cave
- Pneumatic pressure blasts vertical joint
- Shoots geyser-like spray through cliff top
Geos (Tidal Cliff Inlets)
- Entire roof of a long sea cave collapses
- Along a vertical joint or fault line
- Forms narrow, steep-walled tidal Geo
Concordant Coves (Lulworth)
- Hard limestone band runs parallel to coast
- Waves breach a narrow gap in outer wall
- Scoops out circular bay in soft clay behind
Marine Platform Potholes
- Swirling surf eddies on wave-cut platforms
- Rotate hard flint/basalt cobbles
- Drills cylindrical basins into intertidal rock
Coastal Honeycomb Tafoni
- Spray-zone halite ($\text{NaCl}$) crystallization
- Plus alternate wetting & drying
- Carves alveolar honeycomb pits in sandstone
Tidal Range & Platform Width
NARROW
WIDE RAMP
- Micro-tidal ($<2\text{ m}$): Focused notch, narrow platform
- Macro-tidal ($>4\text{ m}$): Broad sloping ramp
- Spreads wave energy across wide zone
Storm & Tsunami Quarrying
- Cyclonic storm waves & tsunamis
- Pluck 50-ton blocks from platform edge
- Toss boulders high onto cliff-top benches
IV. Shoreline Straightening Cycle & Indian Coastal Erosional Landforms
Shoreline Straightening
- Coastal hills & cliffs disappear eventually
- Sediment fills bays with bars & beaches
- Irregular coast becomes straight & smooth (NCERT)
Varkala Cliff (Kerala)
- Only cliff along flat Kerala Malabar coast!
- Mio-Pliocene Warkalli sandstone & laterite
- Declared National Geo-Heritage Monument by GSI
St. Mary's Islands (Karnataka)
- Off Malpe (Udupi) coast in Arabian Sea
- 88 Ma Madagascar-India rift rhyolitic basalt
- Hexagonal columnar sea stacks & platforms!
Konkan, Goa & Karwar Cliffs
- Deccan Trap basalt & laterite sea cliffs
- Harihareshwar & Devgad wave-cut platforms
- Aguada, Vagator & Cabo de Rama headlands
Diu & Saurashtra Sea Caves
- Pleistocene Miliolite limestone coast
- Naida Caves & Chakratirth sea arch (Diu)
- Dwarka & Somnath wave-cut benches
Visakhapatnam Headlands
- Rare rocky promontory on India's East Coast!
- Eastern Ghats Khondalite/Charnockite ridge
- Dolphin's Nose (358 m), Yarada & Thotlakonda arch
Master Coastal Erosional Key
- All 4 NCERT Coastal Erosional forms complete:
- Cliffs, Terraces, Caves & Stacks
- Transitions to Realm 58 (Coastal Depositional)
Core Distinctions
• Wave-Cut Terrace / Platform: Flat or gently sloping rock platform carved directly at the foot of a retreating sea cliff at an elevation above the average height of waves.
• Wave-Built Terrace: Eroded rock debris that falls off the receding cliff gets ground into small fragments, carried offshore by backwash, and deposited below water in front of the wave-cut platform as a Wave-Built Terrace.
Waves focus energy via refraction onto headland flanks: 1) Joints widen into hollows called Sea Caves, 2) Caves on opposite sides of a headland unite to form a Sea Arch, 3) Collapse of the arch/cave roof leaves isolated pillars standing off the coast called Sea Stacks, and 4) Wave undercutting topples the stack into a low intertidal Stump.
3. Three Iconic Indian Coastal Erosional Geo-Heritage Sites (UPSC Prelims)• Varkala Cliff (Kerala): Mio-Pliocene Warkalli sedimentary cliff overlooking the Arabian Sea.
• St. Mary's Islands (Karnataka): Columnar rhyolitic/basaltic sea stacks and wave-cut platforms formed during the $88\text{ Ma}$ India–Madagascar split.
• Dolphin's Nose & Yarada (Visakhapatnam, AP): Rare Eastern Ghats khondalite rocky headland jutting into the Bay of Bengal.
Coastal Erosional Matrix
NCERT Chapter 7 & Indian Exemplar Key