I saw a pattern in the sand yesterday.

I’ve never noticed it before. It’s a kind of cross-hatching pattern on the part of the beach that’s recently been over-washed by waves.

The lines aren’t perpendicular – there’s maybe a 30 or 35 degree angle between them. If you bisect the lines, the result points pretty much straight down the slope.

If you look closely though, the lines aren’t perfectly straight – the channels meander around a bit.

You’ll have to excuse the over-enthusiastic Photoshop work – it was tough to bring out the patterns without abusing the source material.

My guess was that the pattern results from one of the angles in the sand grains’ crystalline structure getting somehow amplified up into a visible pattern, in the same way that table salt crystals are nice and regular. Maybe the “heavier” blunt end of a crystal would tend to be deposited downslope after flowing with the water some distance, leaving the “sharper” end up-slope (a little like the way contact lenses for people with astigmatism spin around to orient themselves correctly), and en masse this tendency causes rivulets to be preferentially dug out along certain angles.

Reading up more about what sand’s actually made of (hint: not that much of it is really silica crystals, at least not at this beach), I think I was wrong.

I hunted around a bit to see if anybody’s got any good theories, and it turns out there’s a bit of a literature about the phenomenon, going back at least to 1887. The patterns seem to be called “rhomboid ripple marks”. The first paper I found was behind a paywall, but it gave me a name to latch onto, which helped me find a recent draft paper by “O. Devauchelle, L. Malverti, E. Lajeunesse, C. Josserand, P.-Y. Lagree, F. Metivier”.

Highlights of the paper are (for me, at least):

• You get these patterns even with sorted, uniform sand (i.e. you don’t need any odd obstacles).
• It sounds as though the researchers reproduced the behaviour in the lab with spherical glass beads, so it’s not about the shape of the individual grains in the way I guessed.
• Similar patterns are seen in sedimentary rocks.
• The patterns have been observed up to the 10 metre scale.
• The authors saw the system (basically, a sandbed across which water was pumped) evolve in three different ways as they varied the parameters they could control:
  • Sometimes, the sand remained flat (I’ve seen this!)
  • Often, the sand formed rhomboid patterns (like those I saw yesterday)
  • Sometimes, the sand formed wave-like ripples (like those I’m used to from other beaches).
• There are beautiful images at the bottom of the PDF.
• The 1887 paper where W. C. Williamson described the phenomenon was titled “On Some Undescribed Tracks of Invertebrate Animals from the Yoredale Rocks, and on some Inorganic Phenomena, Produced on Tidal Shores, Simulating Plant Remains”. Awesome use of Intra-Sentence Capitals. I love it.