Centuries of scouring flood waters washed out the arroyo, or gully, beneath this 110-foot (34 meter) petrified log to form Agate Bridge. The stone log, harder than the sandstone around it, resisted erosion and remained suspended as the softer rock beneath it washed away.

Enthusiastic visitors fascinated by Agate Bridge worked to preserve it through the establishment of Petrified Forest National Monument in 1906. Conservationists felt this ages-old natural bridge needed architectural support an din 1911 erected masonry pillars beneath the log. In 1917 the present concrete span replace the masonry work.

Current National Park Service philosophy allows the natual forces that create unusual features to continue. If discovered today, Agate Bridge would be left in its natural state. Eventually, the natural forces that created Agate Bridge will cause it to fall with or without its supports.