Snowmobile Trail

This was a quick hike I took on the snowmobile trail in Delaware State Forest. The first thing I crossed was a little spot with 3 rock piles to the right of the trail. They appeared to be mostly destroyed or strewn. Here is one:

A stone wall nearby ran northeast, perpendicular to the trail I was taking. It was on the right side of the trail, but it did not continue to the left side of the trail. Further down the trail, a nice pile on boulder:

And near that pile on boulder was this:

Then, further yet, this strange structure that I don't know if it's part of an old foundation or an L shaped stone wall, or something else. It is best described as an L shaped stone wall, all of which fits in this photo:

Beyond that and parallel to the 'back of the L' in the previous photo is a piece of stone wall or rectangular rock pile, I'm not sure which. There is a lot of damage with the tree on it and there is a small dirt service road here, just beyond this pile:

So, I circled back around and started following a stone wall heading back parallel to the trail I followed in. There is a large aperture in the wall:

I couldn't be certain that the aperture was purposely made, or was a product of part of the wall collapsing. However, I really found this one support rock within the aperture interesting. Who would place a stone like this in a rock wall, in that position?