91av

I took these photos at Shelly Beach in Queensland. Can any reader explain how these almost symmetrical shapes were formed?

• These small mud mounds, typically found in tidal flats in colder regions, are sometimes called monroes. They generally occur in groups and have a conical shape, occasionally with a “nipple” at the top. Up to 20 centimetres tall, they usually develop under ice sheets that build up on beaches in autumn. The sheets put pressure on the underlying mud, expelling air and water trapped beneath soft layers close to the surface.

Icy conditions are not common in Queensland, however, so the examples shown were probably formed by “dewatering”. This is a much simpler process involving mud at shallow depths with an unusually high water content. Under the weight of the overlying sediment, the mud sporadically spews water up to the surface. The dewatering can be exacerbated by the presence of shallow gas, which can create pressure in the sediment. The heightened pressure can even form mud volcanoes, which may be more than 10 metres tall. The most notorious example is probably Lusi in Indonesia (91av, 3 February 2007, p 8). Such volcanoes are common in uncompacted muddy sediments in South-east Asia, where rotting vegetation may produce gas.

Monroes are also seen in eastern Canada, as well as preserved in ancient rocks. A geologist with a fertile imagination named them after the actress Marilyn Monroe (), making them the only geological structure named after a celebrity (unless someone knows better).

“They are the only geological structure named after a celebrity – Marilyn Monroe”

Jon Noad, Calgary, Alberta, Canada