Fossilized Seabed

Take a walk on a sandy beach and the chances are you will see some ripples. Small regular ridges of sand or mud, snaking across the surface in parallel lines.  They come in many different sizes, have different cross-sectional shapes, and have whole taxonomy to themselves, with all sorts of obscure ways of describing them in plan-form. 

Beach ripples, curtsy of Mutan, Shutterstock

Different flow conditions create different ripple sets.  For example, symmetrical ripples form below oscillating currents created by waves, while asymmetrical forms are produced by unidirectional currents.  They are not confined to watery surfaces, but you will find them on sand dunes and the dry upper parts of the beach where the wind is the transport agent.  Collectively we referred to these as sedimentary bedform and they can be recognized geologically in cross section.  For example, sediment moves in the direction of the current up the back of the ripple and cascades down the lee side forming an inclined sediment layer called a foreset.  The bedform migrates as sediment is eroded on the up current side and avalanches down the lee.  In this way the inclined layers are preserved in cross-section and can tell a story about the current direction and its velocity.

Many years ago, as an undergraduate I sat on the mudflats close to Aberystwyth happily measuring modern ripples both in cross-section, using a ripple board1, and in plan form by measuring wavelength and bifurcations in plan-form. I loved it!

It is quite rare, although not unknown, for ripples or mega-ripples (big ripples) to be preserved as three-dimensional forms.  Bury the rippled surface in another layer of sediment, without eroding it to much, then in theory you can lift off (by erosion) the covering layer to reveal fossilized seafloor. A big surge of sediment settling out of the water column might do the burying, especially if the original surface was hardened in some way. Exposure, air drying and a bit of salt crystallization to bind the grains would work to do the hardening.

There are some beautiful examples in Snowdonia from the Ordovician.  I first came across these while writing a book about Snowdon’s geology. Imagine a series of volcanic islands with sub-tropical oceans.  Estuaries indent the shoreline and the rivers within bringing lots of sediment to the shallow sea. Explosive volcanoes like those in Snowdonia at the time generate lots of sediment.  Perhaps the best example are those exposed on the southwest side of the Llynnau Mymbry.  These are stunning, large bedforms (water lain dunes in fact) which snake over the inclined rock surface.  An equally impressive example is to be found at the head of the Dolwyddelan Valley.

Large estuarine bedforms from the Ordovician of Snowdonia, curtsy of Andrew Bennett

Recently Richard Hart, a mountaineer, has been sending me photos of many more examples across Snowdonia and I have been pleasantly surprised by just how many examples there are.  If any one is looking for a fantastic undergraduate project, whether you be a geology or geography student, exploring the morphology of these ripples would give an interesting insight into the water dynamics around these island arcs.  And I have never looked on these surfaces for animal footprints which might at a pinch just be there!  You could capture these surfaces in 3D using photogrammetry and explore their morphology. If anyone is interested, please get in touch!

Fossilized seafloor, curtsy of Richard Hart

1 A ripple board is simply rectangle of hardboard covered in graph paper and then sealed in clear plastic. You insert it into the sand and right angles to the ripple crests and then trace with marker pen the surface. This allows you to measure wavelength and ripple height.

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