Somebody was trying to re-create the Erie Canal and this marvelous machine apparently ran out of steam in the middle of the South Forty. Guess somebody forgot to mention that there's no water here....
A Strange-Tripper sent along some spy satellite photo of our little gravel pit, and I was surprised to see this landmark showing up prominently. So, here is ground truth. Point "b" on your photo, if you are following along. Just keep your thumb on your script like this and you won't get lost.