The Rockefeller preserve extends over both sides of Sleepy Hollow Road. This structure stands between the two parts of preserve, and so I guess that technically it’s not part of the preserve. It is, however, pretty much surrounded by it.

I have no idea what it is other than that it stands on a small tributary just before it runs under Sleepy Hollow road into the Pocantico River so I assume it’s associated with a water works of some kind. The style is very similar to that of structures found along the Old Croton Aqueduct, but I don’t believe that the aqueduct, near though it is passes by this location. It looks a lot like the weirs in Ossining and Yonkers. It is fenced off and bears strong warnings no to enter so I assume it’s still in use for something.

After some more exploration I now believe that this is a relieving/access shaft on the NEW croton aqueduct, which opened on opened on July 15, 1890 and runs roughly parallel to the OLD croton aqueduct.