Sir: So ripples on sandy beaches are formed only by wind, not by the sea ('Grains of truth show from riddle of the sands', 29 July). Your Science Correspondent has surely misunderstood the Japanese research.

Of course, some ripples are formed in wind, but they grow also in water currents on beaches or on river beds, even on the ocean floor four kilometres down (no wind there). Certainly the physics of the process is still argued about in terms of sheer stress, bedload, friction, turbulence and so on, but the nature of the two media is never in doubt.

We can observe ripple marks forming in both air and water. Steve Connor should try snorkeling with a face mask off his nearest beach. But ripples would grow equally well in currents of beer or Chateauneuf-du- Pape.

Yours sincerely,

ROBIN G. C. BATHURST

Emeritus Professor

Department of Earth Sciences

Liverpool University