Physiography and Tertiary base levels in the southern Interior Plateau and adjacent areas, southwestern British Columbia

Geomorphology, valley-fill geology, and drainage anomalies indicate that a suite of early Tertiary and older valleys occurs in the study area. Eocene relief and base level of the North Thompson River valley were the same as today,1000m and400mrespectively.The valley predates the middle Eocene. TheThompson River from Kamloops to Martel follows the trend of a broad Eocene valley and is inset 125 to 350 m below the sub-Eocene unconformity. The Thompson River valley from Martel to Lytton and the Nicola, Nahatlatch, and Stein river valleys define a branching, north-flowing drainage pattern that post dates the Eocene Fraser Fault. The Nicola Lake valley is probably Paleocene to Eocene. A former drainage divide existed along the Fraser River valley between Anderson River and Hell's Gate. Base level was about 500 m higher in the Miocene than in the Eocene.