Appearance
1.9 — Water, Ice and Wind: How a Landscape Is Carved
The Ganges delivers roughly a billion tonnes of sediment to the Bay of Bengal every year. Downstream of the river mouth, spread across the floor of the bay and reaching more than 2,000 kilometres south, lies the Bengal Fan — the largest single pile of sediment on the planet, up to 16 kilometres thick.
Every grain of it was once part of a mountain. The Himalaya is being carried, particle by particle, into the Indian Ocean, and the mud your feet sink into on a Bengal riverbank is Everest in transit.
This page is about how that transfer happens, because it is the process that builds every plain, every delta, every beach and every fertile field on Earth.
The three jobs of any erosional agent
Water, ice and wind all do the same three things in sequence, and keeping them separate makes everything else clear.
Erosion is picking material up and cutting into what is left. Transport is carrying it. Deposition is dropping it, which happens whenever the agent loses energy.
Everything follows from that last sentence. A river slows down when it reaches flatter ground, so it drops its load and builds a plain. It slows further when it meets the sea, so it drops the rest and builds a delta. A glacier stops where it melts, so it dumps everything it carried in an unsorted heap. Wind drops sand when it slows behind an obstacle, which is how a dune forms. Find where the energy goes, and you have found where the landscape is being built.
Rivers
A river's power to erode depends steeply on its speed — roughly, the size of particle it can move scales with a high power of velocity, which is why a gentle stream carries silt and the same stream in flood rolls boulders. Most of a river's work is done during the few days a year when it is in flood. That is a general truth about geology and it applies to storms, earthquakes and eruptions equally: the landscape is shaped mainly by rare violent events, not by the everyday average.
How a river cuts down. Chiefly by using its load as tools. Sand and pebbles dragged along the bed grind it away, and pebbles caught in swirling eddies drill circular potholes into solid rock. The water itself contributes by hydraulic action, forcing air into cracks and blowing them apart, and by dissolving soluble rock.
A river cuts down only as far as its base level — the level of the sea, or of a lake it flows into. Below that it has no gradient left and no energy. So when sea level falls, every river in the world is suddenly given new cutting power, and when the land rises, the same thing happens. The Grand Canyon exists because the Colorado Plateau was uplifted while the river kept sawing, so a river that was meandering across a plain found itself trapped in a slot it was cutting downward as fast as the land came up.
Why rivers meander is worth doing properly, because it looks like the river is being decorative and it is not.
Water flowing round a bend is thrown to the outside. That means it flows fastest against the outer bank, so it erodes there and cuts a deep, steep cliff. On the inside of the bend the flow is slow, so it deposits, building a shallow curved beach of sand. Erosion outside, deposition inside — so the bend grows. Every bend deepens itself. Eventually two bends approach each other, the neck of land between them narrows, and one flood cuts straight through. The abandoned loop is sealed off by deposition and becomes an oxbow lake.
This is not a curiosity. It means large rivers move. The Kosi river in Bihar has shifted its course westward by well over 100 kilometres across its fan in the last two centuries, which is why it is called the Sorrow of Bihar, and in 2008 it broke out of its embankment and returned to a channel it had abandoned a century earlier, displacing around three million people. A river on a flat plain is not a line on a map; it is a system that wanders across its floodplain on a timescale of decades.
Floodplains and deltas. When a river overtops its banks, the water leaving the channel slows abruptly, so it drops its coarsest load immediately at the channel edge, building natural levees, and spreads the fine silt across the plain beyond. That silt is the fertility. Every year of flooding renews the soil. This is why the Nile, the Indus, the Ganges, the Yellow River and the Mesopotamian rivers each carry a civilisation on their backs — Part 4 is essentially a tour of five floodplains.
A delta forms where the river meets standing water and stops dead. The shape depends on the fight between the river pushing sediment out and the sea's waves and tides pushing it back. The Nile's is a classic triangle — the Greek letter delta gave the feature its name. The Ganges–Brahmaputra delta is the largest in the world and the most heavily populated, with much of Bangladesh sitting on it barely above sea level.
And dams break the system. A dam traps sediment behind it, so the river below runs sediment-starved and the delta downstream stops being replenished while it continues to compact and subside. The Nile delta has been eroding since the Aswan High Dam was completed in 1970. It is a case where an engineering benefit upstream and a geological cost downstream are both real and both large.
Groundwater and limestone
Rain is mildly acidic (Chapter 1.4), and limestone dissolves in mild acid. Over long periods water seeping through joints in limestone widens them into passages, then into caves, then into whole underground drainage systems. This is karst landscape: sinkholes, disappearing streams, dry valleys, and caverns.
Stalactites and stalagmites form when that dissolved limestone comes back out of solution as the water drips and loses carbon dioxide to the cave air. They grow slowly and in annual bands, which makes them another archive of past climate, like the tree rings of Chapter 1.1.
Karst regions have a practical problem worth knowing: the water goes underground fast and travels through open passages rather than filtering through soil, so groundwater in karst is easily and quickly contaminated. India has extensive karst in the Meghalaya hills — home to some of the longest cave systems in Asia — and in parts of the Deccan and Andhra Pradesh.
Ice
A glacier is not frozen water sitting still. Where snow accumulates faster than it melts, it compacts into ice, and once the ice is thick enough — around 50 metres — its own weight makes the lower layers deform and the whole mass flows downhill.
Ice erodes in two ways that water cannot. It plucks: meltwater seeps into cracks in the bedrock, refreezes, grips the rock, and the moving ice tears whole blocks out. And it abrades: the rock fragments frozen into the base of the glacier are dragged across the bed like sandpaper, leaving parallel scratches called striations that record the direction the ice was moving, sometimes hundreds of thousands of years later.
A glacier's signature shapes are unmistakable, and the reason is that ice fills its valley completely while a river only occupies the bottom of one.
- U-shaped valleys, for the reason in the caption. A drowned one is a fjord.
- Cirques — armchair-shaped hollows bitten into a mountainside where a glacier began.
- Arêtes and horns — knife-edge ridges where two cirques meet back to back, and sharp pyramidal peaks where three or more do. The Matterhorn is a horn.
- Hanging valleys — a small side glacier could not cut as deep as the main one, so when the ice goes its valley is left high up the wall, and its stream now enters as a waterfall.
- Moraines — ridges of unsorted debris, everything from clay to boulders jumbled together, dumped where the ice melted. Unsorted is the tell. Water always sorts material by size; ice never does. Find a jumble and ice put it there.
- Erratics — boulders of a rock type that does not occur anywhere nearby, carried tens or hundreds of kilometres and abandoned. These were the first clue that led nineteenth-century geologists, principally Louis Agassiz, to propose that ice sheets had once covered northern Europe.
Most of the northern hemisphere's landscape is glacial. The Great Lakes of North America are glacially gouged basins. The lakes of Finland, the fjords of Norway, the sandy soils of northern Germany and Poland, the shape of the Scottish Highlands and the boulder clay of much of England were all delivered by ice sheets that retreated only about 12,000 years ago — within the span of human societies, and Chapter 3.6 deals with what that did to people.
Wind and deserts
Wind is a weak agent compared with water, because air is about 800 times less dense than water and can only lift small particles. But in dry places with no vegetation to hold the surface down, it is the only agent there is.
Wind sorts material superbly, because its carrying capacity is so limited. It carries fine dust in suspension over enormous distances — Saharan dust routinely crosses the Atlantic and fertilises the Amazon with phosphorus, which has been measured by satellite. It bounces sand grains along in short hops just above the surface, a motion called saltation, which is why sandblasting damage on desert rocks and vehicles is concentrated in the lowest metre.
Dunes form where moving sand meets an obstacle or a slowdown. Sand climbs the gentle windward slope, spills over the crest, and slides down the steep sheltered face at the angle of rest of dry sand — about 34 degrees. Because sand is continually removed from the back and added to the front, the whole dune migrates downwind while keeping its shape, which is a problem for roads, canals and villages in the Thar and the Sahel.
Loess is the quiet hero of this section. It is wind-blown silt, usually produced by glaciers grinding rock to flour and then picked up off dry outwash plains and carried away. Deposited in blankets tens of metres thick, it makes exceptionally fertile and easily worked soil. The Loess Plateau of northern China is the reason Chinese civilisation began where it did, and loess soils underlie much of the American Midwest's farmland and parts of central Europe. Loess also stands in vertical cliffs when dry and collapses catastrophically when wet, which is why the 1920 Haiyuan earthquake in China triggered loess landslides that killed enormous numbers of people.
Coasts
Waves attack a shore in the same two ways rivers attack a bed: hydraulic force, compressing air in cracks, and abrasion with the sand and shingle they throw. Where a headland sticks out it takes the brunt, so cracks widen into caves, caves cut through into arches, arches collapse into stacks, and stacks are worn down to stumps. Meanwhile the bays between headlands are sheltered, so sand accumulates there. Over long periods the coast straightens itself.
Sand moves along the coast, not just onto and off it. Waves usually approach at an angle, so the swash carries sand up the beach diagonally while the backwash pulls it straight down the slope, and each wave moves every grain a little way along the shore. This is longshore drift, and it is why a groyne built to keep sand on one beach starves the beach downdrift of it, and why harbour breakwaters silt up. Almost every coastal erosion dispute in the world is an argument about interrupted longshore drift.
Soil: where all of this ends up mattering
Soil is weathered rock plus dead organic matter plus living organisms plus air and water, and it is the thin layer that all land food comes from. It forms slowly — commonly cited figures are on the order of a centimetre per century to per millennium, depending on climate and parent rock.
It is lost fast. Ploughing exposes bare soil to rain and wind; removing hedges and trees removes the windbreaks and the roots holding it; overgrazing strips the cover. Erosion rates on cultivated land routinely exceed formation rates by a large factor, and there is nothing subtle about the result. The American Dust Bowl of the 1930s followed the ploughing up of semi-arid grassland whose deep-rooted native grasses had been the only thing holding the soil; a run of drought years then let the wind take it, in storms that darkened the sky in Chicago and New York, and drove hundreds of thousands of people off the land.
India loses very large quantities of topsoil each year to water erosion, concentrated in the ravine lands of the Chambal and the deforested Himalayan and Western Ghats slopes. This is the least dramatic and most consequential geological process in this Part. Chapter 14.6 returns to it.
Where this shows up in your life
Why cities are where they are. Almost every old city sits at a spot the river system chose: the lowest bridging point, the head of navigation, the mouth of a delta, the confluence of two rivers. Kolkata, Patna, Varanasi, Allahabad, Delhi, London, Paris, Cairo and Baghdad are all river-decided.
Why floods are worse than they used to be. Building on floodplains puts people where the river's own deposition system needs to spread. Paving a catchment means rain that used to soak in now runs straight into the river within hours. Embankments raise the river bed by trapping silt, so the water surface climbs above the surrounding town, which is exactly the condition of the Kosi and the Yellow River, and is why their breaches are so catastrophic.
Why your food exists. Every intensively farmed region on Earth is either a floodplain, a delta, a loess deposit, a volcanic soil, or a glacial deposit. All five are places where an erosional agent stopped and dropped what it was carrying.
What the next page covers
We now have the machinery: a planet with a hot interior, drifting plates, rising mountains and relentless erosion. Run it for four and a half billion years and you get a history. Chapter 1.10 lays out that history — the eons, eras and periods, how the calendar was assembled, and the five occasions on which most of the life on Earth was destroyed, including the one that killed 90 percent of species and the one that killed the dinosaurs and left a crater under the Gulf of Mexico.