It’s a small dam, made of stone shaped in a “L” shape, although both sides appear to be equal in length. There’s a trench that goes from the southern-most point towards Hebron Avenue before disappeared into the raised surface.
The purpose of the dam is a mystery. It doesn’t appear on maps available on the internet. It also seems too small to hold enough water to power much anything major but the land surrounding it is rocky and hilly, which seems to rule out the possibility of an irrigation dam.