I've been looking at holes in sandstone for a few years, wondering, not knowing how they were made. Twice, I've written posts about them, here and here, mostly full of questions.
And I've been given some answers on Facebook!
The holes have a name: Tafoni.
|Sandstone rock with empty tafoni, Edgewater beach, 2010|
Tafoni (singular: tafone) are small cave-like features found in granular rock such as sandstone, granite, and sandy-limestone with rounded entrances and smooth concave walls, often connected, adjacent, and/or networked. They often occur in groups that can riddle a hillside, cliff, or other rock formation. They can be found in all climate types, but are most abundant in intertidal areas and semi-arid and arid deserts. (Wikipedia)
"Small cave-like features ..." And "often ... adjacent ..." That describes the ones I find on our beaches. Other sites mention that they can be large, even room-sized, but our shore tafoni are rarely much more than a couple of inches across.
|Often the holes line up around the edge of a flat lump of sandstone. This one is on Stories Beach. (No, I didn't put the small stone on top; that's how I found it.)|
But what causes these? How do they form, and why? Why in these positions?
Explanations of their formation include salt weathering, differential cementation, structural variation in permeability, wetting-drying, and freezing-thawing cycles, variability in lithology, case hardening and core softening, and/or micro-climate changes and variation (that is, moisture availability). (Wikipedia)
That's a partial answer, but seems to leave out any biological factors. So what about snails and limpets? And the anemones in the pits?
|Tafoni full of tiny snails. Willow Point Beach.|
Some researchers believe that, in addition to salt weathering, mollusks and other marine life may also initiate tafoni. They do this by creating small holes in the rocky coastlines, where they attach themselves and extract minerals. The hole grows larger over time until eventually the mollusk, or other organism, drops off. The hole is then left to the elements, like wind, rain, and tidal water. (WorldAtlas)
"Salt weathering". What is that?
Mixing salt cations and water can produce a supersaturated solution. When this solution evaporates, salt crystals precipitate in pores spaces. The resulting crystalline solid precipitated between mineral grains can exert stress and readily cause mineral breakdown. (Tafoni.com Weathering)
Several sites explain this in simple terms. Salt weathering shows up on shore rocks periodically wetted with salt spray. In the intertidal zone, the rocks are underwater most of the time, but spend several hours in the open air daily. In the summer, they are exposed to warm sunlight, and dry out completely.
As the rock dries, the salt crystallizes. These salt crystals expand forcefully enough to create small cavities. In intertidal areas, the drying periods are shorter than on the upper shore, so intertidal tafoni tend to be small.
|Thousands of tiny snails in tafoni, Willow Point Beach|
So here's my idea, so far: the larger sandstone rocks standing above the intertidal floor dry out sooner, and spend more time out of water. This may be part of the reason for the arrangement of the tafoni. Here, salt crystals create small pits in the soft rock. Tiny snails find food in these holes, and dig them deeper; they have the grating radulas for this task. Limpets also bore into rock to create protected sleeping caves.
More weathering occurs. Wind, waves, crystalizing salt, and some chemical reactions depending on the material forming the rock, all may play their part.
And then along come the anemones. Do they enlarge the holes themselves, maybe by chemical means? Or do they just expand to fit the holes they find?
|Anemones in sandstone pits, Edgewater beach, 2010|
That still leaves the question of arrangement; why do the tafoni so often form along the rim of sandstone piles? Why do they sometimes cover the whole rock? Why do they often riddle one rock in a group and leave the rest free?
(The site, Tafoni.com, has much more information, under many headings. Start with Tafoni.com/Definition and go to -Locations to see tafoni on Mars.)