Ripples in Kirkwall
In short: ripples are caused by currents in the sea, you can often spot them in the sand on the coast.
Wait a minute, the ripple marks found here are on a stone. It's solid and doesn't have much to do with sand anymore!
Okay, then we have to dig a little further:
Petrified Seabed
The present stone slab consists of sedimentary rock. When sand is deposited on the sea floor and this is then covered by more and more sand, solid rock is formed over a long period of time - sandstone. Limestone, clay, marl and other mineral layers rich in iron or fossils can also be deposited in this way. Over millions of years, layers of rock are formed that preserve their shapes and inclusions.
Traces in sandstone, such as dinosaur footprints or the ripples discussed here, are still preserved today because clay or loam elements were washed into the shallow water shortly after their formation and covered the traces. The sand then solidified into sandstone through diagenesis. Later, in modern times, the covering layers then crumbled away, so that the sandstone revealed its initial structure to us.
Ripple marks
Currents, such as waves, can not only be seen on the surface, but also affect the seabed and shape it, ripple marks are formed. Depending on their origin, these are differentiated as follows: There are flow ripples and oscillation ripples. A first minimal elevation creates an obstacle in the surface of the sandy bottom, on which grains of sand carried along by the current can accumulate. In order for ripples to form at all, a minimum water depth of about 3 times the ripple height is required.
Flow ripple
Flow ripples, as clearly seen here in a sedimentary rock from Haßberge in Bavaria, Germany, usually run approximately parallel and have an asymmetrical cross-section. This structure occurs when the flow comes mainly from one direction. The current then flows over the resulting ripples, whereby the fine grains of sand are transported over the small hill and remain on the upper end. If too many grains are piled up, the situation becomes unstable and the grains slide down the resulting slope. By repeating this process, layer after layer is formed and a ripple is created. This is also how the shape of the ripple is to be understood: the current flows up the slope of the ripple, at the end of which an edge is formed. The ripples formed here in the image suggest a current flowing down from the top of the stone. The height of the ripples is mostly 3-5 cm, the wavelength 4-60 cm.
Oscillation ripple
Oscillation ripples occur when the current oscillates over the sand, i.e. comes alternately from different, ideally opposite, directions. This condition prevails e.g. a wave, which of course also creates back and forth movements on the ground, runs back in the direction from which it came. The result of these pendulum movements are ripple marks with a symmetrical cross-section, an exact flow direction cannot be determined.
Ripple mark size
According to their size, ripples are divided into three groups:
• Ripples, usually between 3 and 5 cm high and with a wavelength of 4–60 cm
• Large ripple, between 6 cm and 1.5 m high and with a wavelength between 0.6 and 30 m
• Giant or mega ripple, between 1.0 and 8.0 m high, with a wavelength greater than 30 m
Tasks / log conditions for this earthcache
There is no information board on site about the ripple marks. The following tasks can be answered by looking at the stone slab at the coordinates and the listing. Please do not step on the shore stones!
1. What kind of ripples are there? Oscillation ripple or Flow ripple? Compare them with the pictures provided in the listing. If you take a different stone than the one at the given starting coordinates, send a photo of the stone with your answers please.
2. Describe the structure of the ripples and how they were formed.
3. Can a flow direction be determined? Explain.
4. Do we have ripples, large ripples or giant/mega ripples here?
5. Take a photo of you / your GPS / an object that clearly belongs to you along the coastline with the many stones and the sea in the background. Post this photo with your log. Thank you very much!
If you find other stones with ripple marks in this area please take a picture and send it to me with coordinates. I will add them as waypoints.
Just send me the answers to the questions, preferably via the message center! After completing the tasks, you can log immediately. I'll report if there's anything to complain about.
Sources:
- wikipedia.de
- mineralienatlas.de
- spektrum.de