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Water, wind, ice and gravity carry weathered rock away and drop it where they lose energy; a river bend shows both at once.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
In this lesson you put four photographs of a river bend in order before erosion or deposition is named, and then find what you were watching. By the end you will be able to say whether a scene shows material being carried or dropped, name the carrier, read a deposit's size as the carrier's energy and its shape as the distance travelled, and work out how far a cliff has gone from its rate of retreat.
You know that weathering breaks rock down where it lies, and that a sedimentary rock's grain size records how fast the water was moving when the grains were dropped. This lesson is the step between the two: what carries the broken pieces, and what makes it drop them.
Erosion is the carrying away of weathered rock by water, wind, ice or gravity. Transport is the carrying itself, and the load is what is being carried. Deposition is the dropping of the load where the carrier loses energy. A meander is a bend in a river; its outer bank is where the water runs fastest and its inner bank where it runs slowest. Rounding is what a long journey does to a pebble's corners.
Here are four photographs of the same river bend, described because they cannot be shown, and given in no particular order.
Before reading on, put them in order and write down what the river must have done between each one and the next. Where did it take land from? Where did it add land? And what happened, in the end, to the neck?
The answer you are reaching for has two halves. On the outside of a bend the water is fastest and cuts the bank back; on the inside it is slowest and drops sand. Taking and dropping, at the same time, on the two banks of one river — and those two are the whole of this lesson.
Once weathering has broken a rock into pieces, something has to move them, and the moving is erosion. Rivers carry mud, sand and pebbles; the wind carries sand and dust; glaciers carry everything from clay to boulders frozen into the ice; and on a steep enough slope, gravity alone brings fragments down.
How much a carrier can move depends on its energy. A river in flood rolls boulders; the same river in summer moves only sand; a lake moves nothing, and lets even mud settle. So wherever a carrier loses energy — a river slowing on a bend or reaching the sea, wind dropping behind a ridge, ice melting at the snout of a glacier — it drops part of its load. That dropping is deposition, and it is where sediment comes from.
A river bend shows the two side by side. The water runs fastest round the outside of the bend and cuts that bank back; it runs slowest round the inside and drops sand there as a bar. Erosion on one bank, deposition on the other, and the bend grows tighter every year until a flood cuts through the neck and the loop is abandoned as a lake.
The pieces themselves keep a record of the journey. Size says how much energy the carrier had when it dropped them. Shape says how far they came: every knock on the way takes a corner off, so a rounded pebble has travelled far and a sharp-edged fragment has barely moved.
Another way: picture
A river bend seen from above, drawn twice. In the first drawing the bend is gentle, with an arrow on the outer bank labelled cut back and a bar of sand on the inner bank labelled dropped. In the second, forty years on, the bend is nearly a circle with a narrow neck, and a dotted line across the neck shows where the next flood will go.
Another way: steps
To read a scene:
Erosion is used to mean weathering. Weathering breaks; erosion carries. A boulder split by frost on a summit has not been eroded until something moves the halves.
Deposition is read as a kind of erosion. It is the opposite. Erosion takes material away from a place; deposition adds it. A field under a layer of flood mud has gained, not lost.
Erosion is thought to need a river. Waves move sand along every beach; wind builds dunes; glaciers carry more per year than most rivers. Rivers are the commonest carrier, not the only one.
The inside of a bend is thought to be the bank that is cut. The water is slowest there, and slow water drops its load. The outer bank is where the cutting happens.
Where a river meets the sea, a flat fan of mud and sand spreads out into the water, growing a little each year.
Land being added.
The river slows almost to nothing as it enters the sea, loses its energy, and drops everything it was carrying.
Deposition, on a large scale.
The load was eroded from the whole river's valley upstream; the delta is that valley's rock, carried to the coast and dropped.
Erosion far away, deposition here.
Below a crag is a heap of sharp-edged fragments; in the stream bed a little further down, the stones are rounded.
Same rock, two shapes.
The sharp-edged fragments fell straight from the crag and have barely moved: weathered, then a few metres of erosion by gravity.
The rounded stones have been rolled along the stream bed, knocked against each other until their corners were gone: the same rock after a long journey.
Shape is distance travelled.
A river seen from above, flowing from left to right through two bends. A dot marks each bank of each bend. Choose every bank where the river is dropping sand and building the land out.
This task has no paper form; do it on a device.
Four photographs of the same river bend were taken over forty years and then muddled. Put them in the order they were taken, earliest first.
Number the steps in order (write the number in the box):
Three deposits were found. For each, say how fast the water was moving when the pieces were dropped, and how far they had been carried. First: well-rounded pebbles on a bar in a lowland river. Second: angular boulders the size of a chair, piled at the foot of a crag. Third: sand on the inside of a river bend.
| the water when the pieces were dropped | how far they have been carried | |
|---|---|---|
| first | ||
| second | ||
| third |
A cliff of soft rock is being cut back by the sea at about $50$ cm a year. A footpath ran along the cliff top $40$ years ago. How far back has the cliff edge moved since then? Give the answer in metres.
Answer: unit: m / cm / km / mm
A river in flood carries mud past a town. Downstream, where it spreads out over flat fields and slows, the mud settles on the fields. Name the two processes, in the order they happened.
A river seen from above, flowing from left to right through two bends. A dot marks each bank of each bend. Choose every bank where the river is dropping sand and building the land out.
This task has no paper form; do it on a device.
A sandy beach in a bay has been growing at its eastern end and shrinking at its western end for twenty years. No river flows into the bay. The waves usually arrive at an angle, from the west. Which is the best account of what is happening?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Three deposits were found. For each, say how fast the water was moving when the pieces were dropped, and how far they had been carried. First: rounded boulders in the bed of a mountain stream. Second: angular boulders the size of a chair, piled at the foot of a crag. Third: fine clay on the floor of a lake.
| the water when the pieces were dropped | how far they have been carried | |
|---|---|---|
| first | ||
| second | ||
| third |
You can tell erosion from deposition and both from weathering. Say out loud what happens on the two banks of a river bend, and why a rounded pebble and a sharp-edged fragment of the same rock have different stories. Next: reading a whole landscape's shape as evidence of what shaped it.