Not a pure rectangle

I was just out in the woods showing "rectangular mounds" to skeptical people. Every time I tried to argue that the thing was rectangular, I was forced to observe that one corner was slightly off. Three times in a row, looking at three separate mounds, I saw this:

I'll sleep better now. Now that I am telling the truth:) 

Apparently this is not entirely new information. See Figure 4 here: Bulletin of the Massachusetts Archaeological Society, Vol. 73, No. 2 (bridgew.edu)