Doesn't appear to be your normal pile of rocks. There's an inordinate number of rocks with flat faces and a variety of colors.
Can't tell from the picture, ... are some cut with a rock saw ? What about the one on the left ... that cube corner looks like it was man-made ?
If any of these have cut faces, you probably have an old rockhound's pile.
Otherwise, I'm thinking it's a pile of rocks selected to be used in walls, with their flat faces offering ease of placement and surface alignment.
Maybe the leftovers. Are there any nearby walls or chimneys that resemble these rocks ?