Strawberry Mountain lies atop a bed of volcanics and marine sediment deposited during the Early
Jurassic period. This region was later accreted to the
North American Plate. The granite that forms the nearby
Aldrich Mountains intruded into that region during the Early
Cretaceous period. That event formed gold veins throughout the greater
Blue Mountain region. This intrusion was followed by periods of regional volcanism and erosion, representing the last 60 million years. Basalt deposits from the
Columbia River Basalt Group almost entirely surround the base of Strawberry Mountain. 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. This material is typically referred to as the Strawberry Volcanics. Despite having a different chemical composition, some evidence shows that the Strawberry Volcanics are related to the Columbia River Basalt Group. About 100 km3 (24 mi3) of material is estimated to have erupted. Other volcanoes are believed to have existed near the Strawberry Volcanics around the same time period they were active. This region is inferred to have had similar topography to the Oregon
Cascades today. Compression on the north-south axis occurred after these volcanoes became extinct. Erosion by streams and ice age glaciation exposed the inner workings of this volcano as well as early eruptive material. 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. Overlying this gravel is a thick volcanic ash bed. 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. == References ==