Among the curvy lines that you see among the upright slabs as you walk along the Cliffs of Moher, you notice some rocks that exhibit a different pattern. What you see here are Ripple Marks - a sedimentary structure that shows evidence of waves/wind.
Logging Tasks
Send the answers to the following.
- Attach a photo to your log. Choose a slab that contains ripple marks. Do not take a photo of the "squiggly" ones, as that is for GCATA9C.
- What is the shape of the ripple marks here?
- What are the sizes of the crest? From this do you think the currents were gentle or strong?
- Are the ripple marks symmetrical or asymmetrical? From this do you think this was caused by a one way current or two way current?
- If possible, identify the lee and stoss sides and send an image of your finger/arrow pointing in the direction of the flow. If it is a two way flow add a thumbs up.
Ripple Marks
Types of Ripple Marks
Straight
Straight ripple marks all dip in the same direction, and lie in the same plane. These forms of ripples are constructed by unidirectional flow of the current.
Sinuous
Sinuous ripple marks are curvy
Catenary
Catenary ripple marks have a W shape
Lunate
Lunate ripple marks are irregular and have crescent shapes
Reading between the lines
Time to break down the slabs that you see here a little further. Ripples are created by currents such as flowing water or air, we can deduce a few things about the environment it was formed in just from the ripple marks. But first a few key concepts:
I am no artist - but you get the idea
Crest - The point on a wave with the maximum value or height. It is the location at the peak of the wave cycle as shown in picture to the right.
Trough - The opposite of a crest, so the minimum value or height in a wave. It is the location at the very lowest point of a wave cycle also shown in picture to right.
Lee - The lee side has a steeper slope than the stoss. The lee is always on the back side of the ripple, which is also on the opposite side of where the current flow meets the ripple. The current flows down the lee side.
Stoss - The stoss is the side of a wave or ripple that has a gentle slope versus a steeper slope. Current always flows up the stoss side and down the lee side.
Determining wave strength
Speed of the flow impacts the size of the ripples. You can see the crest increases in size as the flow velocity increases.
That's cool and all - but how do I know the flow of the currrent?
Well as mentioned above, if you can identify the stoss (gentler slope) and lee (steeper slope) sides then the idea is the water flows stoss -> lee. But that is only if the ripples are asymmetrical.
Asymmetrical - These are ripple formed when water flows from stoss -> lee. In other words, a one way flow.
Symmetrical - No stoss or lee sides can be identified as the slopes on both sides are equally steep. This is caused by the current flowing back and forth.