Maars and tuff rings...

occur on the southern flank of Newberry Volcano, seen in the distance at the far right in the image above (click to enlarge.) The image shows Fort Rock as viewed from the south, and this phreatomagmatic volcanic feature was created during an eruption into a shallow pluvial lake during the late Pleistocene. Subsequent erosion by wave action breached the south side of the structure and a wave-cut notch, reflecting the former lake's shoreline, is etched around its perimeter.

Hole-in-the-Ground, a related volcanic feature termed a maar is located nearby and is shown in the image below (click to enlarge):

One might notice a small fault that offsets the basalt unit in the far side of the structure.