Geomorphic evidence for Tertiary drainage networks in the southern Coast Mountains, British Columbia

Tertiary drainage networks are reconstructed by mapping drainage anomalies and valley trends on shaded-relief maps of the southern Coast Mountains (NTS 92 G, H, I, J). Geological and structural relationships provide age constraints for the valleys. In the vicinity of Gold Bridge, ancestral Bridge River and Seton River flowed northeast sometime between latest Cretaceous and Eocene time. Strike-slip faulting along the Yalakom and Fraser faults diverted the rivers to the southeast during Eocene time. Lillooet River valley crosscut and captured the headwaters of the ancestral Bridge and Seton rivers. Geomorphic evidence indicates an ancestral Fraser River did not flow south along the trace of the Fraser Fault until sometime after Eocene strike-slip faulting. A dendritic network of streams developed throughout the region in Miocene time, and incised during late Miocene-Pliocene regional uplift.