(screen shot wins every time
)Here you go ... it is (almost) as I thought. As you come up the hill, if you wander about in the weeds, just a bit before the rock tumbler, you get some message like "you find a switch that looks like a rock" (in the screen shot it we be back and to the right a bit).
I am more like the rock, and just took that to be "I triggered some event". The actual switch (rock) is nearer the edge, where the wall looks a bit blockish (where the hand icon is in the screen shot).

I included the offending boulder for reference, as well as the door the switch will open (and close if you hit it again).
As a side note: thanks! In replaying a game to get to this too test this out, I ran into a new monster(s) I had never encountered before, in the swamp: brain borers.
Wiz 8 ... the game that keeps on giving 
Tools for Wizardry(r) 7