One of the glories of spring is the freedom to ramble once more over rocks unencumbered by ice or snow. Often, this is also the time to truly see their more subtle features. With leaf and bud just beginning to open, before grasses and undergrowth take over the forest floor, you will never get better light or less distraction for the eye.

In Quarry Park & Nature Preserve, now is a good time to look for glacial polish, or the scratches left by rocks caught in glacial ice. More enigmatic features, however, also can be found. For instance, on the high ledges above Quarry 4, the granite shows traces of peeling. If you look carefully, scattered across the smooth, glacially rounded outcrops are any number of shallow, flat-bottomed divots. Typically, these are a few inches to a few feet across, with squared, step-like edges about ¼ inch to ½ inch high. A few still hold blocky, dice- to domino-shaped pebbles that broke off the edges, and occasionally, as you walk across them, you can hear flakes still in place flex and grate underfoot against the underlying stone.

This phenomenon is called "exfoliation" and it is rather common in granites. The problem is that, although it looks like an arm with a week-old case of sunburn, there is no immediately obvious way to explain it. Unlike skin, granite does not have a layered structure of cells to guide peeling at the surface. Instead, it is a massive rock built from intergrown mineral crystals. Normally, nothing should be able to steer a crack so precisely just beneath its surface.

Not that granite doesn't break. Most granites have broken many times, usually due to far-off tectonic events, but those joints and fractures tend to be variably spaced, often intersect, and all of them are much older than the last of the glaciers, which actually carved the presently peeling surface.

Like sunburn, then, exfoliation should mark a response to something acting on the surface, not on the rock as a whole. One early interpretation was that changing temperatures were responsible, with repeated thermal expansion and contraction breaking down the rock surface. Unfortunately, rocks are fairly elastic, and no experiment has yet shown anything less than a forge or a fire can split rocks this way, and then the spalling produced isn't quite the same as peeling.

The current explanation (for this style of exfoliation) puts the blame on water instead. If rain or snowmelt were to seep, ever so microscopically, into a granite, it could react with the feldspars there to make clay. That change in mineral structure forces the rock to expand, and over time builds up stress between a weathered surface rind and the unchanged stone farther down. As the stress grows, it eventually forms cracks beneath that swollen, weakened rind, allowing it to flake away and start the process all over again. So, in a way, the rock really does have a blistered and peeling skin — call it a "weathering burn."

This is the opinion of Robert Wichman, a geologist and professor who has explored many of Minnesota's state parks. He can be reached at newsroom@stcloudtimes.com.

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