"As close to the sky as possible, where the slope levels out but the water still flows" - George Hill Upton

That seems to sum up where I expect to find rock piles: at the tops of brooks feeding into navigable waterways. The rule doesn't always work but I have been letting it guide my explorations for the past several months and it has had as many successes as failures - in this Middlesex ex-urban landscape. For example here, here, and here.
I followed this strategy at George's Hill in Upton, going uphill following brooks.

When I got to 'A', I thought: ok this is about where I should find rock piles. I immediately did find rock piles - confirming the search strategy.

It is particularly interesting that in Upton, at 'A', the piles were small (less than 10 ft across) and very old looking - where I am used to seeing larger rectangular mounds with hollows at these brook high-points. This means the placement at the top of a brook persisted through cultural changes in the style of burial mounds.
I saw other piles between 'A' and 'B', of little note. At 'B' I again thought: there could be rock piles here. But I couldn't see any. My feet found one, then I saw a couple others. These were nondescript and might have been evenly spaced in a row.
Here are some pictures of the piles at 'A'. Only about half the actual piles looked slightly rectangular and the faint depression in the middle was only present in a few. I "cherry picked" the best examples to show:
You can see the faint rectangle, and note it is actually a pretty good sized pile:

See the grey rock near the center of the picture? Close up:

Those fluted surfaces are from percussive flaking. This is a core or tool of some kind.
Another:

The snow makes it easier to see. A few more:

These all look faintly rectangular to me. Do you see it or not?