A batholith is a very large (outcrop area greater than 40 square miles and no known floor) intrusive mass of solidified magma, usually granitic in composition. These are emplaced deep in the crust and are irregular in shape.

Exfoliation dome in 1.4-billion-year-old granitic batholith of Rocky Mountain National Park

Proterozoic granitic batholiths are common throughout Colorado’s Rocky Mountains. It was generally thought that the batholiths fell into three age groups: the Routt plutonic suite at about 1.7 billion years ago, the Berthoud plutonic suite at about 1.4 billion, and the Pikes Peak batholith at about 1.0 billion. With more age dating, and more advanced dating techniques, we are discovering that the intrusions were more spread out in time.

Map of granitic batholiths of Colorado

Granitic batholiths of Colorado, from Tweto, 1979.

Decades ago geologists thought that foliated granitic intrusions belonged to the Routt suite and non-foliated granitic plutons belonged to the Berthoud suite. However, the modern dating is showing this not to be the case.

The Diamond face of Longs Peak

The Diamond face of Longs Peak – Highest point of the Estes Park batholith

Rocks of Colorado's youngest batholith form Cathedral Park along Gold Camp Road

Rocks of Colorado’s youngest batholith form Cathedral Park along Gold Camp Road.