Tafone, singular; tafoni, plural! Geologists have a name for the "huecos" at Snowy Mt. that I talked about two weeks ago, and googling will get you wonderful pictures and a fair amount of information. However, these holey boulders are still a bit of a natural history mystery to me and friends who are also trying to figure them out.
In looking for a geologist who would know something about these complex and puzzling cavities, I e-mailed a geologist friend in Stockholm, Sweden. He didn't know anything but his department head did, sending a summary of a Wikipedia article and the word "tafoni." The picture that was with my Ramblings last time somehow made these hollows in the boulder look like rocks in a field. Our tafoni are not as photogenic as many others, but they are the real thing: hollows in solid rock with thin rock walls between them, making them look like honeycomb.
One thing that still seems strange is usually tafoni are in areas that are arid and hot, arid and cold (even on Mars!), or along seacoasts. The Adirondacks are not any of these and never were in the 10,000 years since the boulders came off the source cliff, as far as I know. I do now know that glacier melt had nothing to do with their formation.
In the Adirondacks we don't really have granite; our bedrock that is not anorthosite, a mostly feldspar igneous rock such as in the High Peaks and on the top of Snowy Mt., is "granitic gneiss," a rock that was heated by pressure from the moving of mantle plates till it was plastic (metamorphosed). It has feldspars, quartz, mica, hornblende and other tiny mineral crystals in it. Tafoni can occur in sandstone, limestone, and granite, but apparently not marble, our other common rock around here that weathers easily.
When salt spray or mist lands on rock, then evaporates, salt crystals grow (as sugar crystals do in maple syrup when it sits around a long time) and pry apart the other tiny grains of mineral. With enough time, moisture and drying, big caves can be dug into what seems like hard rock. There can be a tiny amount of regular salt in granite, and there are other salts that can form (gypsum), though I don't know that our gneiss has enough of either to do the wonderful sculpting we have at Snowy Mt. However, there often is a thin coating of a white material on the rock, and I found a thicker layer of white mineral in three small hollows in one boulder.
Recently I took a group of hardy and surprisingly trusting people to refind one glacial erratic near Thirteenth Lake. We did a lot of difficult bushwhacking, but they were great sports, especially the ones who had been lost with me before! But this boulder is easy to find though not to get to. It is different from the other tafoni because it has eroded into about five sections, unlike the others that have virtually no cracks in them. And it is covered with moss whereas the others are barren except for a little green algae.
This tafone also is unique because it is a favorite roosting spot of some tiny bird that sits on a small projection, to judge by the droppings beneath it. I would love to know what kind of bird it is and and see it enshrined in its very special shelter!