Geomorphic Processes: Weathering, Mass Movement, Erosion, Transport and Deposition

• The resulting calcium bicarbonate is highly soluble and washes away, widening joints in the rock.
• Biological Weathering: Breakdown caused by flora and fauna. Plant roots physically pry rock apart, and burrowing animals weaken cliff faces. It also has a chemical component: lichens and mosses secrete chelating agents (organic acids) that dissolve rock minerals.

Mass Movement: Sliding and Slumping

Gravity is constantly trying to pull the landscape downwards, and heavy rainfall is often the only trigger needed to make a cliff collapse. Mass movement occurs when the downward force of gravity exceeds the forces of friction and internal cohesion holding a slope together.

• Sliding: A rapid landslide where rock and soil detach and move downwards along a straight, linear slip plane. The material moves en masse (all together) and remains largely intact until it hits the bottom.
• Rotational slumping: Also known as rotational slip, this is a distinct downhill movement along a curved slip plane.

Rotational slumping is heavily dependent on the underlying geology and the presence of water. It typically happens on coasts where permeable rock (like sandstone) overlies an impermeable layer (like clay).

1. Heavy rain quickly infiltrates the permeable top layer but pools on top of the impermeable clay.
2. The saturated material becomes immensely heavy, increasing gravitational pull.
3. The trapped water lubricates the boundary between the two rock types, drastically reducing friction.
4. If waves erode the base of the cliff (undercutting), the heavy, lubricated material above loses support.
5. The cliff face collapses and rotates backwards along a curved slip plane, leaving a "terraced" profile and depositing a "toe" of material at the beach level.

Erosion: Carving the Land

Every time a storm sends crashing waves against a cliff or floods a river valley, vast amounts of solid rock are carved away. Erosion is the wearing away and active removal of material by a moving force (water, wind, or ice). Rivers shape their channels through downward (vertical) erosion in the upper course and sideways (lateral) erosion in the lower courses.

• Hydraulic action: The sheer, explosive power of water. As waves or fast-flowing rivers hit a rock face, they force air into tiny cracks. The air is violently compressed. When the water retreats, the sudden release of pressure causes the rock to fracture and break apart.
• Abrasion: Often referred to as corrasion, this is the "sandpaper effect". The river or sea uses its sediment load (rocks and sand) as tools, hurling and scraping them against the channel bed or cliff base, grinding it away over time.
• Attrition: Transported rocks and pebbles continuously smash into each other within the water. Crucially, attrition does not erode the river bed or cliff face; it only acts on the sediment itself, making the transported rocks smaller, smoother, and more rounded.
• Solution: Also known as corrosion, this is a chemical form of erosion where weak acids in river water or salts in seawater dissolve soluble minerals, such as calcium carbonate in chalk cliffs.

Transport Mechanisms

Watch a fast-flowing river and you will notice the water is rarely perfectly clear; it acts like a massive conveyor belt for sediment. Transport is entirely dependent on the kinetic energy and velocity of the water.

• Traction: Large boulders and heavy rocks are physically rolled along the riverbed or seabed. This requires the highest energy and usually only occurs during storm events. Traction forms part of the bedload.
• Saltation: Smaller stones, pebbles, and coarse sand are bounced along the bed in a leap-frog motion. The water flow temporarily lifts them before they fall back down.
• Suspension: Very fine, light material (like silt and clay) is constantly held up and carried within the water column, giving rivers a muddy appearance.
• Solution: Dissolved mineral ions are carried invisibly within the water. This requires the least amount of energy.

On the coast, these same transport processes drive longshore drift, moving heavy shingle via traction and saltation, while dragging finer sand along in suspension.

Deposition: Losing Energy

Even the most powerful rivers and ocean currents eventually lose their energy and must drop whatever they are carrying. Deposition occurs when the velocity of the water decreases to the point where its kinetic energy falls below the settling velocity of the sediment. The heaviest material (like boulders) is deposited first, while the finest silt travels the furthest and drops last.

Rivers lose energy and deposit material for several reasons: a reduction in water volume (during dry spells), increased friction from a shallower or rougher channel, a flatter gradient, or the sudden loss of velocity when the river enters the static water of a lake or sea.

Coastal deposition is driven by specific low-energy conditions:

• Constructive waves: These low-frequency waves (6–9 per minute) have a strong swash that pushes sediment up the beach, but a weak backwash. Because the backwash water drains into the porous sand, it lacks the volume and energy to drag sediment back to sea.
• Sheltered areas: Bays, estuaries, and areas behind spits block open ocean currents, heavily increasing friction and slowing the water.
• Flocculation: A unique chemical process occurring in estuaries where fresh river water meets salty seawater. The chemical reaction causes extremely fine clay particles to clump together, gain weight, and sink, forming vast mudflats.