mhosken's picture

Bumps and holes in seashore sands. includes photos of holes and bumps in seahore sand for which no satisfactory explanations are on offer. (The particular section starts "The sands are part of an ecosystem".) Suggestions, corrections and supplementary information will be welcomed by

(There are pages about dunes, rocks and tidal zonation too. Written by a non-expert, corrections and other improvements will be welcomed.)



Peter Skelton's picture

another picture in your gallery

(Copy of e-mail sent directly to Mike):
Dear Mike,
I’m responding to your post on ‘Bumps and holes in seashore sands’ on iSpot. Although not the picture you were actually asking about, I was intrigued by the photo of the dish-like depressions on the sand surface immediately below the sentence ‘Boffins tell us that all forms of fluid flow include so-called "chaotic" elements. It's not clear whether that is supposed to be an explanation or an excuse as to why no explanation is forthcoming !’. I’m a geologist (retired from OU dept.) and what these things reminded me of was the sedimentary structure known as ‘swaley cross stratification’. In the rock record, these are quite common in sands that were deposited in shallow offshore areas, though usually below the depth to which fair-weather waves impinged on the surface. They are interpreted as the effects of storms, with chaotic (yes!) eddies swirling down onto the seafloor to blast out scoops of sand – forming depressions, or ‘swales’ – which then rains back down again to blanket the scoured seafloor, resulting in a stack of successive swales and drapes (forming ‘hummocks’ over the ridges between swales) – as illustrated in the attached photo from the Lower Carboniferous of Howick Bay (…not so far from you, it seems!). Now, I’ve not seen such structures in modern environments, even when diving, but I wonder if that’s what you’ve got here? If so, I would have expected them to have been temporarily preserved relatively low in the intertidal zone following a stormy high tide: does that fit? Very interesting, anyway – and I enjoyed your website.

Best wishes,
Peter Skelton