ID Me: Pebble Poser

by Skip Via
skip@westvalleynaturalists.org

On a walk though the Pig Farm State Trust Lands (see Locations) on December 4, we noticed a phenomenon that made us wonder what was happening with the rocks and soil along the trails. Here’s hoping someone out there can shed some light on the process(es) that were going on.

On this walk we observed hundreds of examples of rocks that appeared to have sunk into the soil along the trails. Some were half an inch below the soil level and others were 4+ inches below the surface. A few weren’t sunk, but the soil around them had separated from the rock’s surface.

A bit of context: The air temperature was around 30F and the soil on the trails was frozen. Temperatures for the preceding several days were below freezing at night and a few degrees above freezing during the day, so there had been some alternating freezing and thawing occurring. Those same days were sunny, so some solar heating may have occurred. The soil was damp/wet due to melting snow in recent weeks.

It seemed like a fairly simple problem, but the more we thought about it the more curious we became. Why did this only occur in some rocks? Some were deeply embedded while their immediate neighbors showed no sign of this phenomenon. It did not seem to matter if the trail was in full sun or deep shade—the embedding occurred everywhere along the trails. The size of the rock didn’t seem to matter—some were golfball-sized and others would have been too large to lift easily.

We didn’t observe this in any of the rocks off of the trails, where there was some ground cover and the soil was not as compacted.

So what process(es) caused this? Here are some possibilities that we discussed:

  • The moist soil expanded by freezing and rose up around the rocks. But then why did this happen with some rocks and not others?
  • The rocks absorbed some heat from the sun and melted the frozen ground beneath them, causing them to sink. But the soil, being generally darker than the rocks, should absorb more heat than the rocks. And again, this only happened to some of the rocks.
  • Alternate freezing and thawing of the soil allowed the rocks to sink. However, just the opposite should have happened. Ground that alternately freezes and thaws pushes rock upward, not downward. That’s why rocks keep appearing in plowed fields and garden beds around here.

Pretty clearly, (to me, anyway) the rocks sunk below the surface. But what caused them to sink, and why did only some rocks sink when their neighbors didn’t?

Any ideas?

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