Little Peoples' Sweat Lodge?

 Suddenly a Stone Sweat Lodge

My friend's original photo:

Note the size of the (Goosefoot Maple?) leaves.

This is a tiny "chamber" similar to larger ones, such as this one below,

accessed at a preview of "Our Hidden Landscapes" at Google Books:

One distinguishing Characteristic of Indigenous Stonework is perhaps the way certain stone structures are "stacked" to resemble, sometimes quite realistically, the scales around the eye of a rattlesnake:

A dramatic photo of a "chamber" in NY, on the Winter Solstice:

Larger preoculars on the left, smaller post oculars to the right,

under a supraocular-like "lintel" stone...

https://wakinguponturtleisland.blogspot.com/2016/12/solstice-serpent-eye.html

Some of you might be thinking, "That's a niche," in Stephanie's photo,

a place to "leave an offering" and that may be so,  

but I think it might also be a small stone sweat lodge,

"Serpent Stacked" to resemble the Eye of the Big Snake Spirit,

perhaps a way of traveling to the Underworld...

Side by side, Little and Big:

"The sacred stone piles on Mohegan Hill are a critical feature of the traditional landscape of Mohegan Hill; they were created by the “Little People” who live deep within the ground of Mohegan Hill. These “Little People” or Makiawisug are the ancient culture heroes of this region. These stone piles also possess powers that protect the Mohegan people from outsiders. Not only do the “Little People” still live within the ground on the Hill and continue to guard the stones, these stone piles are perceived as being made of the bones of Mother Earth and they contain messages that guide generation after generation of Mohegan People. Contemporary Mohegan tribal members make offerings to the “Little People” in hopes that they will continue to protect our Tribe."

http://newenglandfolklore.blogspot.com/2014/06/the-little-people-who-live-under-hill.html

A couple others:

The Snake above is looking to the right:

A little quote:

"When we consider beings supposed to be human, we come to the wigguladumooch-k, or little people, whose footsteps may sometimes be heard in the forest on a still day, though they themselves are rarely seen. They are especially strong in magic power, and will sometimes impart this to the Micmac who wins their friendship. Once in a while, in the woods, one will observe stones piled together so as to make a little house. If you move them and go away, when you return you will find them placed just where they were before you touched them."

Hagar, Stansbury (1896) Micmac Magic and Medicne Journal of American Folklore vol 9, pp. 170-177.

https://archive.org/details/jstor-533400

And then there's this: