It is not so much the shape that makes this building unique, but its on-of-a-kind building material. It is petrified wood! The location is Petrified Forest National Park, where, 225 million years ago, a lot of tall trees were washed into a floodplain, where a mix of silt, mud and volcanic ashes buried the logs. The sediment cut off oxygen and slowed the logs decay. Silica-laden groundwater seeped through the logs and replaced the original wood tissues with silica deposits. Eventually the silica crystallized into quartz, and the logs were preserved as petrified wood. Since the quartz rock of the petrified logs was a lot sturdier than all the other building materials (clay and sand stone), petrified wood was an obvious choice.
The structure in this picture, called Agate House, is a partial reconstruction of an Indian pueblo built here almost ten centuries ago. It was built of petrified wood and sealed with mud mortar. Archaeologists believe the original eight-room pueblo was built between A.D. 1050 and 1300.