Have you ever seen ripples on the sandy beach when the tide was low? They might have looked similar to those in the figure 1:
Figure 1: Ripples exposed in the intertidal zone in Kilkee, Ireland. They were formed by a current (the tide) that was flowing from right to left . Photo: Vane, kompaski.
Or you may have encountered ripples on the sandy sea floor– while swimming or wading in the shallow water of the sandy beach – like in figure 2:
Figure 2: Sea floor ripples
These ripples are usually just a temporary formation – changed or destroyed by waves, new high tide, or changes of wave pattern caused by the change of the wind direction.
TYPES OF RIPPLES AND THEIR FORMATION
The shape of the ripples can usually be divided in two types: asymmetric and symmetric.
Figure 3. Asymmetrical ripples
Asymmetric ripples show a gently rising side (stoss side) and a short, steeper inclined side (lee side). The sediment is dragged and eroded from the stoss side until it reaches the crest and deposits on the lee side, which is downstream with respect to the current. Ripple crests do not stay at one place - the continuous removal of sediment from the stoss side and the re-deposition on the lee side causes the ripple crest to migrate in the direction of the current.
Figure 4. Symmetrical ripples
Not all bedforms are the result of a single and dominant current. Symmetric ripples are formed by bidirectional currents: currents that move in one direction and then in the opposite one. Does it ring a bell? Waves! Waves cause ripples to be symmetric because both sides of the ripple become alternatively sites of erosion and deposition while water moves back and forth.
RIPPLE MARKS IN THE STONE
The physical processes that are active in the world today occurred in the past and will continue to occur in the future. And ripples can tell us a lot about the past of our planet!
Figure 5: 300-million-year-old ripples (in the stone) next to 300-minute old ripples (in the sand). Photo credits Ian Kane. The image is powerful, because it shows the basic principle of geological actualism.
Sedimentologists study and analyse bedforms, like ripples, in present-day shorelines, river systems, deserts, and in deep marine environments like submarine fans to understand past environments. It is pretty obvious, when walking on a strand, to tell ripples were shaped by waves along the coast or by blowing wind on sand dunes, but would you be able to tell how they developed in rocks, without actually seeing the ambient where they formed? The first and most obvious information we can get from the presence of ripples in sedimentary rocks is that a current must have been present- either a water current or a blowing wind. Their crests are always oriented perpendicular to the current that formed them, telling us what the direction of currents in past environments was.
Their shape, size and symmetry depend on the type of sedimentary process that is associated with their formation.
Recognizing asymmetric ripples tells us immediately where the flow was directed. We can, for example, reconstruct the direction of a river, or a marine current, or the dominant wind in sandstone that deposited millions of years ago. Recognizing wave ripples can tell us whether an ancient sandstone deposited on a shoreline rather than on a riverbank or a dune field.
SPANISH POINT RIPPLE MARKS
The coastal section at Spanish Point consists of sandstones, siltstones and mudstones of the Upper Carboniferous (Namurian) – these rocks were formed approximately 317 to 330 million years ago. At that time land that is now Ireland was much closer to the equator. Muddy rivers deposited sand and silt in the sea. Conditions were similar as in the delta of the river Mississippi in the Golf of Mexico today.
Sedimentary structures are well preserved here at Spanish point and include cross-bedding, cross-laminations and symmetrical wave ripples.
Figure 6: Symmetrical wave ripples in the rock formations of Spanish point; photo: Vane, kompaski.
Figure 7: Symmetrical wave ripples and cross-bedding in the rock formations of Spanish point; For the information about the cross-bedding please see the reference No. 4; photo: Vane, kompaski.
VISITING SPANISH POINT
There is a public car park and no access issues to the coastal rock exposures. Visitors should exercise caution with tides on a rocky coastline.
LOGGING TASKS
At the header coordinates:
1. Measure the distance between neighbouring crests (ridges) of the ripples. What is the average distance?
2. Are all the rock layers with ripple marks aligned in the same direction? Or are they lying in the different angels to each other?
At the info board (see reference point): The information board has been destroyed / removed . Please let the cache owner know if it has been replaced.
3. As sea level rose and fell due to glaciation, marine shales were deposited and these often contain fossils called ______ (ten letters).
IMPORTANT!df
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REFERENCES
1. Blogs of European Geosciences Union; Features from the field: Ripple Marks, Samuele Papeschi, 2019
2. Banner Rocks, The Geological Heritage of County Claire; Matthew Parkes, Claire County Council.
3. Claire County Geological Site Report for Spanish Point.
4. Website: nagelmyers.wordpress.com/rensselaer-falls; The sedimentary structures.