The initial Cambrian record in most of the project area consists of medial BonniaðOlenellus Zone and younger Lower Cambrian strata overlying a Precambrian surface of various ages. The Richardson Trough came into existence during an Early Cambrian transgression, and continued to function as a negative entity, separating the Mackenzie Platform to the east from the Yukon Block to the west. On the Mackenzie Platform, a thin Lower and Middle Cambrian clastic succession (Macdougal Group) is overlain by Upper Cambrian and younger dolostone (Franklin Mountain Formation). The Richardson Trough contains Lower Cambrian limestone (Illtyd Formation, new name) that grades upward though massive mounds into Middle? and Upper Cambrian basinal platy limestone and shale (Road River Formation). In the Yukon Block, siltstone and clean, thick bedded Lower Cambrian limestone of the Illtyd Formation is overlain by Middle Cambrian fine to coarse grained clastics of the Slats Creek Formation (new name). Slats Creek deposition reflects block faulting, whereas the overlying Taiga Formation (new name), comprising Middle and lower Upper Cambrian clastics and carbonates, represents the process of a return to more uniform depositional conditions. The Taiga Formation is overlain by an unnamed thick succession of Upper Cambrian through Lower Devonian clean carbonates ( Db), not unlike the clean carbonate within the Illtyd Formation below. The BritishðBarn Trough is north of the Yukon Block, and separated from it by the Kaltag Fault. There the Cambrian is within a poorly dated succession of volcanics and basinal clastics. The volcanics can be lithologically correlated with strata in Alaska which contain Lower and Upper Cambrian fossils. Recent reconnaissance mapping has outlined several areas underlain by basinal argillites containing Lower(?) Cambrian trace fossils. In the southern part of the region, outcrops which have been mapped as Road River Formation may contain some Cambrian strata.