Rocks. I think that's the trail, passing through the crevice.

Entrance to Bear Gulch Cave. Here's the National Park Service description of the cave.

Inside the cave, before I figured out how to use my camera's "night mode"...

Same picture, with night mode turned on. The arrows are marking the trail. The splotch of yellowish light in the middle is from the flashlight I was using to aim the camera.

Looking back down at the cave exit.

The trail goes up.

The reservoir.

More looking back down.

A weird rock.

Lots of rocks.

And some more rocks.

Oh look, rocks. This time with the trail carved right into the stone.

If you're more than three feet tall, you have to lean sideways for this part.