Seemingly drilled into the schist of Inwood Hill by some kind of large-bore drill, this is actually a glacial pothole, scoured out by the mighty power of swirling water and abrasive stones during the heady days of the Wisconsin glaciation. The diameter is a little over a foot and a half.
The heights of Inwood, very like a whale from the vantage point of the Hudson, are the northern prong of the Fort Washington Ridge. Geologically, this rock is known as the Manhattan Formation, made of mica and hornblende schist. (“Manhattan’s gneiss, but full of schist” goes the immortal line.) This is the good, hard stuff, the serious, no-nonsense rock, but everything has its weaknesses, and time grinds it all up, as in the mills of the gods.