The eastern Klamath belt contains the fault-bounded Yreka, Trinity, and eastern Klamath terranes. The Yreka terrane comprises Lower Cambrian to Middle Devonian or younger igneous, metamorphic, and sedimentary rocks. The Trinity terrane consists of the Trinity ultramafic-mafic complex of Ordovician to Silurian age and an amphibolitic gabbro unit of Early Cambrian age. The eastern Klamath terrane bears igneous and sedimentary rocks of Early Devonian to Middle Jurassic age.

Stratigraphic and intrusive relations imply that the Yreka and Trinity terranes were amalgamated by Early to Middle? Devonian time and that the Trinity terrane was the basement on which the eastern Klamath terrane formed.

The lower Paleozoic rocks of the Yreka and eastern Klamath terranes are interpreted to represent the remnants of an Early Cambrian arc overlain by part of an Ordovician to Devonian arc-trench complex that faced west to northwest (present coordinates) in Late Ordovician and Early to Middle Devonian time. The Trinity complex may have formed in a marginal or back-arc basin northeast to southeast of the Lovers Leap–Gregg Ranch portion of the arc in Ordovician to Silurian? time (prior to existence of the eastern Klamath terrane).

The biogeographic affinities of eight groups of early Paleozoic fossils, taken as a whole, demonstrate that the eastern Klamath belt was close enough to North America in the Middle Ordovician to Middle Devonian for faunal communication to occur, in some cases at the species level. This evidence and the presence in the belt of some coarse-grained strata possibly derived from a continent indicate that the belt may have been relatively near to North America in Silurian or Devonian time, yet its location is obscure.