Rock pile in wall corner but with hollows

Here is where the trouble begins:It is reasonably common to see a stone wall corner, closed off and filled with a triangular pile of rocks. I have always liked these but continue to suspect they are the work of tidy field clearing activities. But usually when I see one (like here or here) they are filled up. So what is with the hollows in this one (where FFC is pointing)? Either an internal chamber fell in, someone made the hole afterwards, or the process of filling in the triangle from discarded field stone was not completed. I am going to go with the latter because this whole area of Carlisle is packed with stone walls that have been added to in this way: an extra retaining wall, fill, loads of stone filling the space.

One very typical feature of field clearing is that all sizes of rock get cleared but they tend to be sorted by size in the process. In other words, first they clear the biggest rocks and dump them. Then they clear smaller and smaller rocks and dump them, leading to separate cluster/pile/groups of stones with somewhat homogenous rock sizes. So you see sub-piles of bigger rocks next to or underneath sub-piles of smaller ones. That is about the most distinct characteristic of field clearing. Another very important characteristic is a nearby field that is relatively free of rocks. Another characteristic is a general messiness to the pile construction. But I am starting to think the messiness is not guaranteed. I am going to make the case that one, pretty standard, technique was to create a stone wall enclosure and dump rocks into it. Or maybe the enclosing wall was added later. That is consistent with the wall corner pile above. Hope I am not wrong but here goes with classifying characteristics for field clearing piles:

• Piles of small rocks next to piles of big rocks
• next to a cleared field
• messy (often at the angle of repose)
• occasionally enclosed with a more tidy wall.
  

Right or wrong, according to this classifier, the pile in the wall corner is from field clearing.

So, to illustrate, here is a massive stone wall that likely was made from field clearing:
Note the smaller sized fill and the nature of the outer walls. My eyes need to become accustomed to and recognize this.