Sunset Park is buttressed by a rough stone retaining wall that has become the home of numerous lifeforms. Above is the southwest-facing flank. 
Here’s the northeast wall, along 41st St. That’s where all the following were found:
The presence of lichen, which doesn’t tolerate pollution, means the air here is relatively good. Indeed, elevated near the top of the Harbor Hill Moraine, the park catches the harbor breezes very nicely.
There are numerous clumps of Scotch Moss (Sagina subulata), which isn’t a moss but rather a flowering plant.
Haven’t yet figured out which fern this is. A spleenwort perhaps? 
Virginia Creeper (Parthenocissus quinquefolia).
The caterpillar of the Sycamore Tussock Moth (Halysidota larrisii). Wikipedia says these can cause hives; this Auburn entomology page says nix to that, while listing other “stinging” caterpillars.’
“Stone wall, Sunset Park ……… $50,000” from the May 10, 1906, edition of The City Record. Would love to know where these stones came from.
I think that’s Ebony Spleenwort, but I’m not sure.
Thanks for the clue, Deborah. I have access to a “Fern Finder” that’s keyed to some very close-in details, so I just need find some time to examine further. Indeed, the whole wall is ripe for a survey…
Excellent Urban Expose. The western wall of my childhood home was so buttressed by big rusticated blocks. Like cliffs with declivities and creases hosting a mosaic of life from lichen and moss to insect and spider, even snake.
That wall looks like trap rock (basalt, or diabase depending on how deeply it formed). It is entirely absent in Brooklyn except for scattered pieces here and there as part of the terminal moraine. It was likely imported from one of the Watchung formations in New Jersey where it was quarried extensively until the early 1900s. It is still quarried on a much smaller scale in some places.
Thanks, Doug. Good to know. It does have a basaltiness to it.