[4][5] Strawberry Mountain lies atop a bed of volcanics and marine sediment deposited during the Early Jurassic period.

The current edifice is made up of andesitic volcanic material erupted locally about 16 to 14 million years ago, a timeframe that overlaps with the most active period of the Columbia River Basalt Group.

Erosion by streams and ice age glaciation exposed the inner workings of this volcano as well as early eruptive material.

[9] Much of the material eroded from Strawberry Mountain now lies as gravel in part of the John Day River Valley and is known as the Rattlesnake Formation.

The fossilized remains of several animals ranging from the Eocene to the Miocene are preserved within the Rattlesnake Formation, including several now-extinct species of horse such as Eohippus and Merychippus.