Sediment records not only what it is made of, but how it was laid down. Ripples, cracks and slanting layers frozen into sedimentary rock tell which way water flowed, whether waves were sloshing, whether the mud dried in the air, and even which way was up when the layer formed.
- Cross-beds: sloping layers inside a thicker bed, left as ripples or dunes migrated. Sand rolls up the gentle upstream side and avalanches down the steep front, so the cross-beds dip in the direction the current or wind was moving.
- Ripple marks: lopsided (asymmetric) ripples come from a current flowing one way, their steep side facing downstream. Even, symmetric ripples come from waves sloshing back and forth.
- Mud cracks: wet mud dried in the air, shrank and cracked into polygons, wide at the top and narrowing downward.
- Graded beds: coarse at the bottom, fine at the top, left when a muddy flow (such as an underwater turbidity current) slowed and its grains settled, biggest first.
Why it works
Each structure is made by a process that has a direction in time and space. Gravity settles coarse grains before fine ones, so a graded bed's coarse end was at the bottom; mud dries from the top, so cracks narrow downward; cross-beds are cut off at their tops and curve out at their bottoms. That lets geologists tell whether a layer has been turned upside down, which happens in tight folds.
Key idea
Cross-beds and asymmetric ripples show which way a current flowed (toward their steep side); symmetric ripples show waves; mud cracks show drying in the air; graded beds and mud cracks show which way was up.
Worked examples
Worked example: Which way did it flow?
Asymmetric ripples in a sandstone have their steep sides facing southeast. Which way did the current flow?
- Sand climbs the gentle side and tumbles down the steep side.
- So the current flowed toward the steep side: to the southeast.
Answer: Toward the southeast.
Worked example: Which way is up?
In a cliff, a layer of sandstone is fine-grained at the bottom and coarse at the top. What happened?
- A graded bed settles coarse first, so it should be coarse at the bottom.
- This one is the other way around: the rocks have been folded and overturned.
Answer: The layer is upside down.
Common mistake
Reading every ripple the same way is a common slip. Symmetric ripples, with matching slopes, come from waves moving water back and forth; they do not show a current direction. Only lopsided ripples do, with the steep side downstream. Check the two slopes before you read a direction.