Site Density - a Chelmsford example

It is easier to find rock piles in some places than in others. One of the basic questions to wrestle with is: why here but not there? So I left work and on the way home went to explore a part of northern Chelmsford. There was nothing but quarried rock and no signs of ceremonialism. Why not?
It is different in southern Chelmsford. Here is a map fragment showing red outlines where I planned to explore in southern Chelmsford and blue lines for places I have now visited. There are rock piles in three of these and houses in the fourth.

Why did I circle these places? Because of the topography and the green color of the map. So it seems that site density is probably high in this part of Chelmsford. I would even be willing to place bets on the remaining outlines in southern Chelmsford. But was northern Chelmsford empty because of a coincidence of that place, or was the original site density low in that part of Chelmsford? A bigger sample of sites up there might help answer the question.