Shaping the Surface

Geology & Plate Tectonics Fundamentals

Course 1 · Chapter 6 · Surface Processes: Weathering, Rivers and Ice

Plate tectonics and volcanoes build the land; weather, water and ice take it apart and carry it away. Every pebble on a beach and every grain of sand is a small piece of a larger rock that has been broken down and moved. Hutton saw this as part of a great cycle: the land wears away, the sediment is laid down in the sea, and in time it is lifted again to become new land. This chapter covers the three stages (weathering, erosion and deposition), the landforms of rivers, glaciers and coasts, and the way ice shaped much of Scotland. Figures and dates are from general knowledge and flagged "check" where they matter.

Weathering, Erosion, Deposition

Weathering

The breaking down of rock in place, without much movement. Physical weathering splits rock, for instance when water freezes in cracks and expands (freeze-thaw). Chemical weathering changes the minerals, as when slightly acidic rain dissolves limestone (check).

Erosion

The removal and transport of broken material by water, ice, wind or gravity. Weathering and erosion work together: weathering prepares the material and erosion carries it away.

Deposition

The laying down of material when the carrying agent loses energy, for instance where a river slows as it enters the sea. Deposits that are buried and cemented become sedimentary rock (Chapter 2).

Rock type matters Hard, resistant rocks such as granite and quartzite tend to form high ground, while softer rocks such as shale and mudstone are worn into valleys. This is why a map of rock types often looks like a map of hills and lowlands. The relationship is not simple: climate, structure (Chapter 5) and the history of ice also play a part.

Rivers

Stage (check)Typical featuresMain process
Upper courseSteep, V-shaped valleys, rapids, waterfallsDownward erosion
Middle courseWider valley, meanders beginningErosion and transport, sideways cutting
Lower courseBroad floodplain, meanders, oxbow lakes, levees, deltaDeposition
  • Load. Rivers carry material dissolved in the water, as suspended fine particles, and rolled or bounced along the bed. Faster water carries bigger pieces, which is why floods move boulders.
  • Meanders. Water flows faster on the outside of a bend, which erodes it, and slower on the inside, where sediment builds up. Over time bends migrate and sometimes get cut off as oxbow lakes.
  • Floodplains. Fertile flat land made by repeated floods, which is why many cities and farms are on them and why flood management matters (check).

Glaciers

Feature (check)How it forms
U-shaped valleyA glacier widens and deepens a former river valley
Corrie (cirque)An armchair-shaped hollow cut by ice at the head of a valley
Arête and pyramidal peakSharp ridges and horns left between corries
Hanging valley and waterfallA small side valley left high above the main glacial valley
MoraineRidge or sheet of rock debris dropped by melting ice
DrumlinSmooth, elongated hill of glacial deposit, shaped in the direction of ice flow
ErraticA boulder carried far from its source and left stranded
Fjord or sea lochA glacial valley flooded by the sea

Ice sheets covered much of northern Britain during the last glacial period, which ended about 11,700 years ago (check). The weight of ice also pressed the land down, and the crust is still rising slowly in Scotland as it recovers, while parts of southern England are sinking (check).

Coasts and Wind

  • Waves erode cliffs by hydraulic action and abrasion, forming wave-cut platforms, caves, arches and stacks as headlands are cut back (check).
  • Longshore drift moves sand and shingle along a coast in a zigzag path, building beaches, spits and bars.
  • Wind moves sand in deserts and on dunes, and shapes the surface by abrasion and by sorting grains (check).
  • Sea-level change drowned river valleys and left raised beaches where the land rose. Scotland has both (check).
A British comparison Edinburgh's skyline is glacial. Castle Rock and the Royal Mile form a classic crag and tail: ice flowing from the west was blocked by the hard volcanic plug of the Castle Rock and left a ridge of softer rock and deposit trailing behind it (check). The Highlands are full of corries, U-shaped glens and sea lochs, and Loch Lomond lies along the Highland Boundary Fault (Chapter 5, check). The Firth of Forth is a drowned and glacially deepened valley. A walk from the Castle down the Royal Mile is a walk down the tail.
Check the details Glacial dates and extents are revised as new dating methods arrive, and the details of how individual landforms formed are often debated. Check specific claims about a location against British Geological Survey or NatureScot (formerly Scottish Natural Heritage) resources. Coasts, rivers and cliffs can be dangerous: heed tide tables, local warnings and rockfall signs.

Exercises

Exercise 1

Explain the difference between weathering, erosion and deposition, using one example of each from a river's journey to the sea.

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Exercise 2

Describe how you could tell, from a photograph, whether a valley was cut by a river or by a glacier. Name three clues.

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Exercise 3

Explain what a raised beach tells you about the history of a coast, and why some parts of Britain are rising while others sink.

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Quick Reference

  • Weathering: breakdown in place (physical and chemical)
  • Erosion and deposition: removal and laying down by water, ice, wind and gravity
  • Rivers: V-shaped valleys, meanders, floodplains, deltas
  • Glaciers: corries, U-shaped valleys, moraines, drumlins, fjords
  • Scotland: glaciated landscape, crag and tail, rising land (check)