Cobble Depressions in Sand

After some field ventures through brush and ravines, the best way back to the car was via a forest road versus back tracking through the brushy route I had descended. The upper forest road I hiked in on was across sitltstone and sandstone and the weathered rock was impressively slippery. The lower road was across alpine glacial outwash dominated by sand, cobbles and boulders and was easy walking and only a bit further.  

I was cognizant of deer, elk and cougar tacks on the road so I was paying attention to the road surface. I also thought it best not to run given the presence cougars and did occasionally look behind me to be sure no confused cougars thought mean easy mark.  

The depressions on the surface of this road section were not from animals, but were from rocks that had sunk into road surface.

The ground had been frozen earlier in the day. My take on this phenomenon was that the water around the sand with some silt caused the surface layer of sand to expand while the larger rocks took longer to freeze and likely did not freeze at all. The sandy soil expanded upward, leaving the cobbles in depressions. The sand around the cobbles was soft and non compact from the freezing expansion. Deeper freezes can cause the reverse and push stones up above the surface. The weather the night before was in the low 20s but had been 50s during the day.

Seeing the dynamic surface soils process was a good reminder of foundation depths for frost protection. Probably would want a deeper footing depth in this chilly east slope of the Cascades valley.