The Ashgillian (Upper Ordovician) Montgomery Limestone occurs as slide blocks in melange of the Shoo Fly Complex, northern Sierra Nevada, northern California. Brachiopods and sphinctozoan sponges from the Montgomery Limestone have closest biogeographic ties to coeval faunas of the eastern Klamath Mountains (Yreka terrane), and in the case of the brachiopods, to east-central Alaska (Jones Ridge). The latter was part of North America in the Ordovician. A small collection of Montgomery rugose corals yielded one species that is known elsewhere only in the Yreka terrane and in northern Maine. Montgomery tabulate corals have affinities with contemporaneous faunas of the Yreka terrane, northern Europe/Asia, Australia, and eastern North America. The apparent absence of similar tabulate taxa in western North America may be an artifact of incomplete collecting. As a whole, the biogeographic data indicate that the Montgomery Limestone was deposited close enough to Ordovician North America for faunal interchange to occur, and during its deposition was probably relatively near that continent. A comparison of lithologic units of the Shoo Fly Complex with those of the Yreka terrane indicates that some units in each area have no counterparts in the other (e.g., schist of Skookum Gulch in the Yreka terrane), and other units have general similarities in age (where known) and lithology, but differ in detail. The Yreka terrane has been interpreted as the remnants of an Early Cambrian arc and Ordovician-Devonian arc–fore-arc–accretionary prism, and the Shoo Fly Complex as a fragment of a Devonian or older accretionary wedge. Available biogeographic and stratigraphic data can be reasonably explained, as has been done by earlier authors, by a paleogeography in which the Yreka terrane and Shoo Fly Complex were parts of the same arc-trench system but were situated at different points along the strike of the arc. Lateral changes along strike in tectonic conditions and source areas could account for the observed disparities.

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.