Here is a familiar pattern: you are at the bottom of a narrow valley and see a stone wall climbing up the side. You see a large rock next to the wall, maybe 30 yards up the side, and you think: if that rock is split, I'll expect a wedge. For example pictures #3 and #4 here. Out for walk with FFC at the Wheeler Lane end of the Nashoba Brook Conservation Land in Acton, just across the foot bridge. When he started uphill saying he wanted to show me something, I could guess we were about to see a split-wedged rock. Yup it is split:Yup it is wedged: (We noted, in passing, that significant effort would be needed to pry up this rock and get that wedge in there.) This seems like a very familiar location. Maybe I remembered it from a previous visit?
By the way, here is a picture of FFC.