Quaternary stratigraphy and geomorphology of the Lower Thompson Valley , British Columbia
Creators: name: Anderton, Lesley Jean
Publisher: name: University of British Columbia
Year: 2011

description: The Thompson Valley between Spences Bridge and Lytton, where it joins the Fraser, merits attention because of the abundance of clear sections of late glacial and postglacial deposits in road and rail cuts, and its position as an important transition zone between the glacial lake deposition of the southern Interior and the dominantly fluvial aggradation of the Fraser. Most attention was paid to the stratigraphy of road and rail cuts, as the only morphological features preserved in the steep and narrow valley are terraces, fans, landslides and talus slopes. Quaternary deposits yield little evidence of the depositional environment prior to the last major advance of ice, but there is a good record of conditions during and following deglaciation. The last ice sheet, which wasted away by down-melting, left a thin mantle of till over the uplands and till deposits up to 50 ft. thick in the valley. The Lower Thompson Valley, close to the glacier sources of the Coast Mountains, was one of the last areas to be free of ice. Consequently major lakes developed in the Thompson and Nicola Valleys and were forced to drain into the Okanagan Valley. When the ice plug south of Spences Bridge failed, some time before 9,000 B.P., drainage was resumed down the Thompson Valley to the Fraser. During deglaciation, up to 300 ft, of silts and deltaic gravels were deposited in the l,230 ft. stage of Lake Deadman, which was dammed by ice south of Spences Bridge. Between Skoonka Creek and Seddall, respectively 4 and 7 miles south of Spences Bridge, are deposits of collapsed silt and flow till and it is suggested that an ice plug remained here separating lacustrine deposition north of Skoonka Creeks from fluvial aggradation south of Seddall, where the valley was by then largely ice-free. Aggradation, which was due to the large supply of material from lateral sources resulting from recent glaciation and weathering, was extremely rapid; more than 500 ft. of cross-bedded and horizontal gravel and sand were deposited in probably little more than 1,000 years. Aggradation ceased well before 7,530 B.P. With amelioration of the climate and stabilization of the slopes, the load of the river was considerably reduced and it degraded its former valley fill creating a series of terraces, with a veneer of imbricate cobbles, from 500 ft. down to 30 ft. above present river level. Most terraces are non-cyclic, but paired terraces, approximately l4O ft. above the river, mark a period of relative stability prior to 7,530 B.P. The river was probably within 50 ft. of its present level by 6,600 B.P., and since then downcutting appears to have proceeded relatively slowly. At least three phases of alluvial fan formation occurred during this period of dominant degradation, the terraces acting as temporary base levels for the mudflows. Mazama ash, deposited on the terraces and fans approximately 6,600 years ago, provides a useful marker horizon. Apart from slow down-cutting by the river and some alluvial fan formation, the dominant processes affecting the topography since 6,600 B.P have been landsliding and slumping. The largest slide, the Drynoch Earthflow, has been active for at least 3,175 years and slumping due to seepage and river erosion has occurred intermittently along the river banks.