A page for every year
How Do Rocks End Up in Stripes?
A cliff of banded rock is a stack of surfaces, each one a piece of ground that was lying in the open long ago.
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Step 1 of 5
Settling material lies flat.
Anything settling out of still water spreads into a level sheet. That is why sedimentary layers start out horizontal, whatever they look like now.
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Step 2 of 5
Burying turns it into rock.
Layer after layer piles on top, squeezing the water out and pressing the grains together. Minerals crystallise in the gaps and cement the whole thing solid.
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Step 3 of 5
The oldest is always at the bottom.
Nothing can be laid down under something already there. So in undisturbed layers, order is age, and it can be read like a stack of dated pages.
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Step 4 of 5
Each band records its own conditions.
Coarse grains mean fast water. Fine mud means still. Salt means a sea drying up. Read across a cliff and the climate and geography change as you go.
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Step 5 of 5
Tilted layers mean something happened later.
Layers that are folded, tipped or broken were laid down flat and moved afterwards. The shape they are in now is a record of what happened after they set.
A gap in the stack is called an unconformity, and it can mean millions of years of rock were laid down and then worn away before the next layer arrived.
The short version
Layered rock is a stack of old surfaces, laid down flat and in order, with the conditions of each time written into its band.
Try it yourself
Shake soil, sand and gravel in a jar of water and let it settle. It bands itself, coarsest at the bottom.