Fort Rock is the iconic image of central Oregon. This formation is a tuff ring, formed when magma interacts with wet soil. A few hundred feet lower in elevation than the Hole in the ground, this magmatic encounter may have been with surface water. Wikipedia has a whole description of what happens. Whatever happened, it's an interesting formation. Oh, and don't forget the Fort Rock Valley Historical Homestead Museum.
|Fort Rock from the Highway||The Homestead Museum|