Previous age estimates of Fort Rock ranged upwards to 1.8 million years.
Recently, the age of Fort Rock has been estimated at 50,000 to 100,000 years.
This coincides with a period of time when large pluvial lakes filled the valleys of central Oregon and much of the Great Basin of the western United States.
At its maximum, the water in Fort Rock Lake was estimated to cover nearly 900 square miles (2,300 km²) and was about 150 feet (46 m) deep where the Fort Rock tuff ring formed.

The extensive terrace on the side of Fort Rock marks one lakeshore about 14,000 years ago.
Even higher water levels are recorded on the tuff cliffs and at one point only the tops of the tuff ring were exposed as rocky islands in this inland sea.
An age of about 21,000 years ago has been found for this highest lake level.
Previous age estimates of Fort Rock ranged upwards to 1.8 million years. Recently, the age of Fort Rock has been estimated at 50,000 to 100,000 years. This coincides with a period of time when large pluvial lakes filled the valleys of central Oregon and much of the Great Basin of the western United States. At its maximum, the water in Fort Rock Lake was estimated to cover nearly 900 square miles (2,300 km²) and was about 150 feet (46 m) deep where the Fort Rock tuff ring formed.
The extensive terrace on the side of Fort Rock marks one lakeshore about 14,000 years ago. Even higher water levels are recorded on the tuff cliffs and at one point only the tops of the tuff ring were exposed as rocky islands in this inland sea. An age of about 21,000 years ago has been found for this highest lake level.
Fort Rock