This one may be at the outer limits of altar-dom, depending on just how fluid you feel like being. I would've included it no matter what, but what really clinched it was the "inscription" (Can caves have inscriptions? Message, graffiti, art, etc.) inside the cave, which reads:
This itself is religious / It demarcates a sacred space
It's a note that's frustrating for a few reasons, the first of which is, don't pretentiously tell me via vandalism what is a sacred space. The second, keeping the first reason in mind, is that it's true. Caves have been sacred spaces since humans began to formulate both art and sacredness, which Werner Herzog can narrate to you in Cave of Forgotten Dreams. Not only that, but a major, major point of altars is the ability to demarcate sacred space as you choose, more or less where you choose, by the mere decision to arrange objects and text.
This particular cave is in Two Harbors, on Catalina Island. Mom came to visit from Vermont, and treated us to a two-night stay. In the last hours before getting the ferry back to LA, we rock-scrambled just a teensy bit off the sandy beach in the harbor. The cave is small, with shells and rocks people have tucked into its walls' natural nooks and crannies, and someone lined its edge with rocks.