Diagonal line of rock-on-rocks going up a hillside. Rattlesnake Hill, Bolton MA

I went to explore the valley at Rattlesnake Hill and when I got to where the water started, there was a rock-on-rock:

Not in good shape, my main reason for wanting to get up the hillside a bit, while going back down the valley, was to get out of the deeper snow for easier footing. It turned out I kept running into rock-on-rocks in a sequence something like this:

Here are the other three:

 

 

The first of these three I only spotted because I sat down to catch my breath. So there could be others structures I did not notice, elsewhere on the slope. But I have seen this before, at a the gully that divides Proctor Hill in Littleton. I was puzzled then by the same question as now: what do these little "cairns" mark? 

I was not aware of following a trail as I walked, just of choosing a direction of least effort.