Deposition

All this erosion means radically altered landscapes. But because the Earth is a closed system, it also means that material is being dumped elsewhere. Once the ice, wind or water carrying eroded material away from its original location no longer has the energy to transport it further, the process known to geologists as deposition occurs.

If a glacier dumps material, it creates chaotic-looking terrain which we shall examine in more detail in Chapter 7. More organized and satisfying to the eye are the landforms produced by the wind (aeolian, in the jargon). Where wind speeds fall below the level at which the wind can carry the material that has been eroded away, it is deposited, usually in dunes. Dunes typically form in long lines at right angles to the wind. In coastal areas, they are sometimes crescent-shaped because of the influence of onshore winds. The wind can push dunes along at a rate of several metres a year, as they are made of unconsolidated sand.

As most erosion occurs under the influence of wrater, most rocks are created by the deposition of material from water. As with the wind, the sediment-carrying power of water goes up rapidly with its speed, and falls as fast if it slows. Look at any river taking a bend and the effect will be apparent. On the outside of the bend, you will see the water moving at its fastest, and eroding the river bank as it goes. On the inside, where the water is slower-moving, you will see material being deposited. The deposits could be anything from chunky boulders to fine sand, depending what the river has flowed through on its journey so far. Such banks are also fine spots to find fossils and washed-down archaeological remains. Over time, the combination of erosion at the outer bank and deposition at the inner one will shift the watercourse across the valley. Many a tonne of concrete has been poured by civil engineers in attempts to keep rivers from shifting in this way.

The general rule that slower water means deposition also applies when rivers meet, with the smaller river often slowing and forming sandbanks and other structures. This process is seen at its most spectacular when a river meets the sea. The load of sediment it drops can form sandbars, shingle bars or small offshore islands that form a significant hazard to shipping. As the area just offshore is a high-energy environment of waves and surf, these structures are often eroded about as fast as they are built up, so an average sand grain in one of them may not stick around long before moving on.

When larger rivers arrive at the sea, they can form a delta. The first delta to be described was that of the Nile, which carries exceptionally large amounts of sediment to the Mediterranean. Over time the river has deposited material hundreds of metres deep and steadily cut new channels called distributaries to keep making its way to the sea.

However, even a river delta may not suffice to absorb the sediment borne by a major river. Often it forms barrier islands, long islands parallel to the coast, from material which has been carried down the river and then been slowed to a halt when it reached the sea. As the name suggests, these are valuable for protecting the coast and the people who live there from severe storms. When conditions get rough, ships take refuge on their landward side.

Deposition in the sea is rarely a simple process. In high-energy areas with large waves and storms, rocky material such as pebbles tends to dominate. But it there is no such material available from nearby erosion, the coast will tend to be eroded back. In quieter waters you can get sandy or even finer silty deposits, perhaps on a lee shore with no big storms, or when there are offshore islands, reels or sandbanks in place to keep the water calm.

However, deposition on an energetic shore is not for ever. Instead, it is subject to “longshore drift”, the process whereby material keeps moving along the coast in the general direction of the wind and current. All those groynes put up along bathing beaches to keep the sand in place are there to slow’ this process down. That is why the sand builds up on one side of them, in the direction of its travel, and you get a large drop on the other side.

The deep oceans are also the site of deposition, especially the abyssal plains that dominate the deep ocean beyond the continental shelves, typically 4-6km below’ sea level. In areas such as the North Atlantic, where the ocean is opening and ocean floor is not being destroyed, the abyssal plain can build up kilometres of fine sediment. Some of it is the remains of life from nearer the surface, as bits of everything from plankton to whales rain slowly down. More significant is the sediment that arrives at the plain in massive subsea flows running down the continental slope and across the plain. The resulting deposits are called turbidites, aptly because of the turbid nature of the currents that emplaced them.