Physical Processes Shaping Landscapes

In lowland areas, the glaciers lost energy and deposited their load, creating flat plains. They left behind massive deposits of glacial till (unsorted, unstratified debris ranging from clay to boulders). At the edge of the ice, meltwater rivers deposited glacial outwash (sorted layers of sand and gravel). Mounds of accumulated debris formed different types of moraine.

Weathering Processes

You can easily snap a piece of school chalk in half, but trying to snap a piece of granite is impossible. Because rocks have different strengths, they break down in completely different ways.

Weathering is the breakdown of rock in its original place (in situ). Crucially, weathering does NOT involve the transportation of material; if the material is moved by a river or glacier, it is erosion, not weathering. Along with mass movement, weathering is a key element of the sub-aerial processes that shape valley sides from above.

In upland areas, mechanical weathering dominates. Water enters cracks in resistant rocks. When temperatures drop below $0^\circ C$, the water freezes and expands by approximately 9%, forcing the rock apart. Repeated freeze-thaw cycles shatter the rock into angular fragments, which fall to create large scree slopes.

In lowland areas, chemical weathering dominates. Slightly acidic rainwater dissolves permeable sedimentary rocks like limestone and chalk. This process, known as carbonation, acts faster in warmer, wetter climates. The chemical process occurs in two steps:

1. $CO_2 + H_2O \rightarrow H_2CO_3$ (Carbonic Acid)
2. $H_2CO_3 + CaCO_3 \rightarrow Ca(HCO_3)_2$ (Calcium Bicarbonate, which is soluble and washes away)

Biological weathering (where tree roots or burrowing animals physically break rocks apart) is also more common in lowlands due to thicker soils and denser vegetation.

Fluvial (River) Processes

A river uses approximately 95% of its energy just to overcome friction from its bed and banks. Only 5% of its energy is available for fluvial processes (erosion, transportation, and deposition).

In upland areas (the upper course), rivers focus their limited energy downwards. This vertical erosion cuts deep, narrow channels, forming steep V-shaped valleys. The river erodes its bed via hydraulic action (the sheer force of water compressing air into cracks) and abrasion (the river's load grinding against the bed). The stones themselves collide and become rounder through a process called attrition.

In lowland areas (the middle and lower course), rivers use lateral erosion (sideways) to widen their channels and valley floors. The thalweg (the line of fastest flow) swings toward the outside of meander bends, undercutting the banks. When a lowland river floods, its velocity drops rapidly, causing it to deposit fine, fertile silt called alluvium across its floodplain.

To understand a river's power to shape the landscape, geographers calculate its discharge:

Where:

• $Q$ = Discharge (cumecs, $m^3/s$)
• $V$ = Velocity ($m/s$)
• $A$ = Cross-sectional area (Width $\times$ Mean Depth, $m^2$)

Slope Processes and Mass Movement

Mass movement is the downhill movement of weathered rock, mud, or soil under the influence of gravity. It is triggered when the downward gravitational force exceeds the resisting forces of internal friction (typically on any slope over $5^\circ$).

In rugged upland areas where slopes exceed $40^\circ$, freeze-thaw weathering fragments hard rock. Gravity pulls these loose fragments down rapidly in a rockfall, accumulating at the base.

In undulating lowland areas, softer sedimentary rocks like clay are highly prone to saturation. Heavy rainfall increases the weight of the soil and lubricates the contact point with impermeable clay beneath. Gravity overcomes friction, causing a section of land to slip downward along a curved slip plane. This is known as slumping (or rotational slip). Gentle lowland slopes also experience soil creep, the extremely slow downhill movement of soil particles caused by gravity combined with wetting/drying cycles. Extreme saturation on straight slip planes can also trigger a rapid landslide.

The Interaction of Processes: Shaping the Landscape

Landforms are rarely created by a single event; they are the result of multiple physical processes overlapping and interacting over thousands of years.

In the Lake District (an upland landscape), tectonic uplift initially created steep gradients. During the Ice Age, glaciers used these steep gradients to carve massive U-shaped valleys. Today, smaller post-glacial rivers flow through these valley floors as misfit streams (so named because they are far too small to have eroded the enormous valley themselves). Meanwhile, ongoing sub-aerial freeze-thaw weathering breaks rock off the high peaks, and mass movement pulls it down, burying the valley edges in relict landscapes of scree.

In The Weald and South Downs (lowland landscapes), the interactions are very different. Millions of years ago, tectonic pressure folded layers of chalk and clay into a massive dome. Chemical weathering and lateral fluvial erosion wore away the soft clay much faster than the resistant chalk. During the Pleistocene, permafrost made the remaining chalk completely impermeable, allowing surface meltwater to erode deep valleys. When the climate warmed, the ground thawed and the chalk became permeable again. The surface water sank underground, leaving behind empty dry valleys.

When explaining these interactions, it is crucial to establish a causal chain. For example: Heavy rain leads to saturation, which triggers mass movement, resulting in sediment falling into a river, which the river then uses as "tools" to increase its rate of abrasion.