Round Mountain is a dormant volcanic cinder cone located in the Clear Lake Volcanic Field in the eastern end of the High Valley.

It last erupted approximately 8,500–10,000 years ago and filled the valley with volcanic debris, raising the valley level to 400 feet above Clear Lake.

[1] The Clear Lake Volcanic Field is the site of Late Pliocene to early Holocene activity and consists of lava domes, cinder cones, and maars[2] with eruptive products varying from basalt to rhyolite.

The Geologic History of the surrounding area including the volcano (which is covered with heavy concentrations of chaparral, oak, Pacific madrone, manzanita, western white pine and other plant species of the California coastal mountains) has been studied and documented by the USGS and prominent geologists as far back as 1938.

The volcano is credited with the creation of the High Valley through repeated eruptions the last of which is thought to have occurred 8,500–10,000 years ago.