A friend spotted these unusual pointed sand forms (pictured) on a beach in northern Lake Michigan recently. My first thought was that the shapes had resulted from rain or waves that had been breaking over the beach, but I鈥檓 still at a loss to explain how the shapes might have formed.
There are a variety of possible answers here that have two things in common: a liquid capable of binding sand together and erosion. Readers disagree about what the liquid is and whether it comes from above or below ground 鈥� Ed
鈥� The beach surface may look bare, but there is a thriving ecosystem in muddier layers beneath. Grazing marine worms burrow through it and then excrete waste onto the surface. Gut secretions and microbial films bind each miniature 鈥渕ud volcano鈥� slightly better than the bare sand around. Gentle erosion does the rest.
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Nik Kelly, Liverpool, UK
鈥� The picture shows a layer of wet compacted sand at the base of a series of sand pillars reminiscent of upside-down mushrooms. These shapes are formed as raindrops or spray from the lake fall onto a layer of dry sand supported by compacted wet sand underneath. As the drops hit the dry sand, they penetrate it and flow downwards, forming a vertical cylinder of moist sand.
When the liquid front of each droplet reaches the wet compacted sand underneath, the liquid begins to flow across the surface of the dry-wet sand interface, as the sand beneath is saturated. If the wind picks up at some point, it can blow away the surrounding loose, dry sand, exposing the mushroom shapes.
Similar upside-down mushroom shapes can be formed by placing drops of water on lactose powders and sieving away the unwetted powders.
Karen Hapgood, Seven Hills, New South Wales, Australia
鈥� The structures pictured appear to be what sedimentologists refer to as 鈥渕ud volcanoes鈥�, and are formed when a waterlogged layer of fine sand or silt is covered by a layer of coarser sand. The weight of the coarse sand increases fluid pressure in the watery layer underneath. Because the wet sand is fluid, it can escape upwards, making a series of muddy piles once it breaks the surface.
This can be seen in the photograph: some of the shapes are darker, and therefore wetter, than the surrounding sand and are superimposed on the wind-formed ripples, showing that they have been deposited on top at a later stage. Such structures are sometimes found preserved in rock formations.
Matt Carrol, Nottingham, UK
鈥� The shapes look to be urination concretions from an animal that crouches to pee. My cats make these in their litter box. The small central cone is the original point of urination and the larger cone-shaped area is the wetted patch of sand. The urine then dries, and the solids glue the wetted portion into a lasting monument as the wind blows the surrounding dry sand away from the area. These concretions also may resist rain and waves to a greater degree than loose sand.
Bill Jackson, Toronto, Canada
鈥� I found very similar shapes when working in a field camp in the Tunisian Sahara. They were created when I had to go out in the night for a pee, and used an area of soft sand behind my tent. The urine, if one kept one鈥檚 aim straight, appeared to vanish down a hole, but must have encountered harder, less porous layers below the surface and then spread out.
鈥淭he shapes look to be urination concretions from an animal that crouches to pee. The urine dries and the solids glue the wetted portion into a monument鈥�
It must also have acted as a cementing material, because a few weeks later, following a severe sandstorm which blew away all the soft surface sand, these 鈥淢exican hats鈥� remained very noticeable.
鈥淭hese could be 鈥榤ud volcanoes鈥� formed when a waterlogged layer of fine sand is covered by a layer of coarser sand. Its weight pushes up the fluid wet sand鈥�
Bob Fryer, Comrie, Perthshire, UK