Marker piles fading into the slope

[Still at High Ridge in Westminster] Making a straight line traverse back to my car, I came across one rock pile and it turned out to be part of a decrepit site. Sites must be quite common to show up when you are not even looking for them. 

Saw this pile and then looked around more carefully for others nearby - about six of them were visible on the slope above. They were in lines and evenly spaced but almost erased.

Trying to enhance the locations:

Quite a steep slope:

The nicest preserved pile was at the top (just visible at upper left of previous picture), in detail:

(To be clear: I believe this is the same type of site as the Spring Hill "grid", which I assume is a calendar.)
A few moments later, I was over the hill and steps from my car. Here is a view towards the parking lot: