Once the solid rock at the Earth's surface has been weathered and eroded, rivers, ice and wind will transport the fragmental (clastic) material to a different area where it will be deposited as sediment, the foundation for a new generation of sedimentary (and perhaps metamorphic) rocks in another turn of the Rock Cycle.

Rivers have enough energy from the effects of gravity to transport a vast 'load' of material, as

bed load suspended load solution load

During transport, the particles of the bed load become smaller and more rounder due to attrition, the wearing away of the grains. When the current slackens, the heavier fragments are deposited first.

Only the lightest particles, carried in suspension, will be transported as far as the sea.

The solution load is deposited in conditions of strong evaporation, for example in the ephemeral playa lakes of deserts.

Transport and deposition are important in determining the texture of a sediment.

By comparing the textures and structures of modern sediments with those of consolidated sedimentary rocks, we can suggest the conditions under which these ancient sediments were deposited and build up a picture of the geography of the area at that time. This in turn provides evidence about continental drift.

This comparison between modern and ancient sediments is helped by examining the characteristic sedimentary structures which are formed as the sediment is deposited.

Currents of water or air build the sediment into sand banks or sand dunes in which the internal structure of layers slope 'downstream'.

This cross bedding is preserved in some dune sandstones, and is used to work out the direction of the prevailing wind (Britain lay within the north-east trade wind belt 200 million years ago) or the location of the mountains from which rivers flowed (indicating the possible position of former plate boundaries).

On a smaller scale, wind or water ruffles the surface of the sand into ripples.

When the current is predominantly in one direction, the ripples are asymmetrical in profile, showing the direction of the current.

Symmetrical ripples show that the water is oscillating, as for sea waves; a sandstone with symmetrical ripple marks was originally deposited on a beach. This indicates the position of a former coastline, millions of years ago.

Other sedimentary structures are mentioned below.

Interpretation of these structure relies on the Principle of Uniformitarianism - that the laws of Physics and Chemistry have been constant over time.

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Sedimentary structures are not only invaluable indicators of the geographical conditions which prevailed at the time of deposition but they can also be used to intepret the folding which the rocks may have suffered during later earth movements. By the very nature of their formation, sedimentary structures have a clearly defined 'top' and 'bottom'. Structures preserved in sedimentary rocks can therefore show whether these rocks are still the original way up or whether they have been turned upside down by the immense forces of mountain building at a destructive margin when two tectonic plates moved towards each other.

For example:

• sand banks and dunes may have their tops eroded by the current of wind or water;
• mud-cracks open upwards to the warm air which which dried them out
• in graded bedding, the heaviest particles fall to the bottom

Look at the images of various sedimentary structures. First, identify what kind of structures they are and decide what made them. Which ones are inverted - turned upside down by earth movements since they were deposited as sediments? Which ones are the right way up?

Sediment remains unconsolidated for many years, and is often reworked and eroded. Some, however, will remain relatively unscathed while it is buried beneath younger sediment deposited on top. As more and more of this younger sediment builds up (the overburden), the pressure causes the original sediment to be compressed and reduced in volume. The grains are forced together and water in the pore spaces is squeezed out. The sediment is hardened.

Meanwhile, fluids flowing through the sediment may deposit minerals in the pore spaces which, as it precipitates, binds the grains together. The sediment is cemented.

So

sand becomes sandstone gravel becomes conglomerate mud becomes shale or mudstone

This compaction and cementation change the sediment into a sedimentary rock, a process called lithification. Once consolidated, the rock has a better chance of survival, even though it may later be eroded or metamorphosed etc in another part of the Rock Cycle.

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The relationship between the sediment, its sedimentary structures and its fossils can be used to describe the environment of deposition , an interpretation of the geography and climate of the area at the time of deposition. Knowing where mountains, coastlines and oceans were located, and how their positions changed over time, leads once again to the development of continental drift and plate tectonics.

Areas of marine environments, where deposition is dominant, tend to produce sediment of great extent and thickness, often fossiliferous.

Terrestrial environments produce sediment with different characteristics, and a full range of sedimentary structures, but fewer fossils.

The Study Topics

• Lithification and Diagenesis
• Sedimentary Structures
• Transport and Deposition
• Way-up Criteria
• Marine Environments
• Terrestrial Environments

are included in the Geopix "Sedimentation" Study Guide. The Guide also contains a selection of 70 multiple choice questions (with answers!) which help to test understanding of Sedimentation.

To order the "Sedimentation" Study Guide please visit the Geopix Online Store