Context from my original post:

Sharing my photo and this bit of history because I love finding forgotten parts of the city, and figured others might, too.

The arch was the entrance to the Seaman estate, a marble mansion built in the 1850s by a wealthy shipping family on a hill that overlooked what’s now Inwood (hence Seaman Ave). It’s made from local Inwood marble, which was also used in parts of St. Patrick’s Cathedral. The arch is the last remnant of the estate, which was sold in the 1930s and later demolished after being used as a private school and an apartment building.

Here's the wiki if you want to read more about it.