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Grain size records how fast the water was moving and the layers record the order things happened in, so a column of strata is a sequence of events.
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 read a column of four layers before any rule is given, and then find the two rules you were using. By the end you will be able to name the rock a sediment makes from its grain size, say how fast the water was moving when it was dropped, put the events a column records in order from the bottom up, and turn a layer's thickness into a time when a rate is supplied.
You can tell a sedimentary rock by its texture: grains stuck together, usually in layers, sometimes with shells. You know that a fast stream can roll a pebble along while a still pond lets the finest silt settle to the bottom. This lesson turns those two facts into a way of reading a rock as a record of the water that made it.
Sediment is loose material — pebbles, sand, mud, shells — before it becomes rock. Grain size is how big the pieces are. A layer, or bed, is one sheet of sediment laid down at one time, and a stack of them is strata. Deposition is the dropping of sediment by water, wind or ice. Cementing is what glues buried grains into rock, as water leaves minerals between them.
A cliff shows four layers, and here they are from the bottom up.
Before reading on, answer three questions and write your answers down. Which layer was laid first? What kind of water can carry a pebble, and what kind lets mud settle? And what does it take for a layer of shells to end up on top of a layer of mud?
Most people get the first one right and hesitate on the second. The rule you are reaching for is the whole lesson: the faster the water, the bigger the grain it can carry, so the grain size in each layer is the speed of the water written down — and the layers, read upwards, are the story in order.
Water carries sediment, and how much it can carry depends on how fast it is moving. A river in flood rolls pebbles along its bed. A river flowing steadily carries sand and drops the pebbles. Water that is nearly still — a lake, a deep sea floor — carries nothing but the finest mud, and lets even that settle.
So when the sediment turns to rock, the grain size is a record:
And the layers are a second record. Each bed was laid on top of the one before it, so in an undisturbed stack the bottom layer is the oldest and the top layer is the youngest. Read a column from the bottom up and you have a sequence of events: the water was fast, then steady, then still, then the sea came in. A sedimentary rock is a diary written in grains.
Another way: picture
A tall column drawn as it lies in the ground. At the bottom, rounded pebbles; above them, a band of sand; above that, plain fine mud; on top, a layer full of little fan-shaped shells. Beside each band, an arrow labelled with the water that dropped it, from a rushing river at the bottom to a calm sea at the top.
Another way: steps
To read a column of strata:
The top layer is taken for the oldest. It is the newest: it was laid last, on top of everything else. The only way an older layer sits on a younger one is if the whole stack has been turned over, and a Grade 6 column is not.
A thick layer is taken to mean a long time. Thickness is how much sediment arrived, not how long it took. A flood can drop a metre of sand in a day; a deep sea floor can take a thousand years to gather a centimetre of mud. The transfer item gives you a rate, and only with a rate does thickness become time.
Fine grains are taken to be heavy. The finest grains are the lightest and the last to settle, which is exactly why they need the stillest water. Mud over sand means the water slowed.
The conglomerate is at the bottom, so it was laid first. Pebbles need fast water: a river in flood, or a rushing mountain stream.
Bottom first; grain size to speed.
The mudstone is on top, so it came later, and mud settles only in nearly still water.
Same two readings for the second layer.
So the fast river was replaced by still water — a lake, perhaps, where the river was dammed. Two layers, one sentence of history.
Sandstone below, limestone full of shells above. The sand came first, from steadily moving water.
Shells pile up on a sea floor, so after the sand was down the sea covered this place.
The layer says where the sea was, not where it is.
And the cliff is inland now, so the sea has since gone away again. The rock records a coastline that no longer exists.
A cliff shows four layers. From the bottom up: rounded pebbles in a sandy cement; sandstone; mudstone; and limestone full of shells. Put the events that made the cliff in the order they happened, earliest first.
Number the steps in order (write the number in the box):
Three samples of loose sediment were sieved. Sample A has grains about $0.2$ mm across, sample B about $0.02$ mm, and sample C about $50$ mm. For each, name the rock it will make once it is cemented, and say what the water was doing when it was dropped.
| rock it will make | the water that dropped it | |
|---|---|---|
| sample A | ||
| sample B | ||
| sample C |
The picture is a column of layered rock, drawn as it lies in the ground. Mark the layer laid down by water moving steadily, fast enough to carry sand and no more.
This task has no paper form; do it on a device.
In a cliff, a layer of mudstone lies directly on top of a layer of sandstone. What changed while these two layers were being laid down?
A cliff shows four layers. From the bottom up: rounded pebbles in a sandy cement; sandstone; mudstone; and limestone full of shells. Put the events that made the cliff in the order they happened, earliest first.
Number the steps in order (write the number in the box):
Engineers checking a reservoir know that mud settles on its floor at about $1$ mm a year. A core drilled from the floor shows a mud layer $25$ cm thick. How many years did that layer take to build up?
Answer: years
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Three samples of loose sediment were sieved. Sample A has grains about $0.02$ mm across, sample B about $1$ mm, and sample C about $20$ mm. For each, name the rock it will make once it is cemented, and say what the water was doing when it was dropped.
| rock it will make | the water that dropped it | |
|---|---|---|
| sample A | ||
| sample B | ||
| sample C |
You can read a column of strata as a sequence of events and a grain size as a water speed. Say out loud what sand under mud under shells tells you, in order, and why the top layer is the youngest. Next: what heat and pressure do to a rock that is already a rock.