Surficial geology, Clearwater, British Columbia

The Clearwater map area consists of gently rolling to rugged uplands at about 1200 m in elevation, with summits reaching over 1900 m in three separate areas. The uplands are dissected into three roughly triangular sectors by the North Thompson and Clearwater Rivers in the east and a broad valley running from the northwest corner of the map area to the southeast. A maximum of about 1500 m of topographic relief occurs between the floor of the North Thompson valley and the rugged mountains to the east, part the Shushwap Highlands. The bedrock is very diverse with Mesozoic granitic rock intruding into sedimentary and volcanic formations of Paleozoic age. Moreover, flat lying or gently dipping Tertiary lavas usually obscure the older rocks. Most of the ridges in the area are rounded and appear to be glacially overtopped in the past. Long glacial flutings are usually discernible on rock surfaces and areas of discontinuous till cover on local uplands. Till cover is usually thicker down valley slopes where it forms continuous blankets. Several ice-flow directions are prominent in the area. In the northern sector, between the North Thompson River and Mann Creek valley, the dominant flow was from the north likely completely inundating Grizzly Mountain and Clearwater Peak (>1900 m above sea level). In the southwest sector of the map area, the dominant flow was to the southeast. This is approximately along the axis of the Mann Creek valley and a broad valley running parallel to it, now partially occupied by Taweel Lake. A third major flow direction from the northeast, is found east of North Thompson River. Ice flows contrary to the main flow directions are found in small areas within the main flow zones. These flows may be remnants of older ice advances or they may be short-lived deglacial flows of thinner ice tongues where the local topography exerted a greater influence on the glaciers. Unfortunately, because the contrary flows are found in isolation, it was not possible to determine their relative age. Deglaciation in the northwest part of the map area likely progressed to the north and to the northwest in the western part of the map area. However, in the eastern half of the map area, the ice margin likely receded to the east up the North Thompson River toward the accumulation areas on the Cariboo Mountains. Ice retreat was probably dynamic for the most part, however, numerous kettle lakes and areas of hummocky moraine, mainly over the plateau area south of Taweel Lake, suggests that the some stagnant ice masses remained. Numerous short-lived meltwater channels were produced as base levels continually dropped. There were likely numerous proglacial lakes but glaciolacustrine deposits are rare or limited to isolated sand bodies. However, remnant ice lobes in the larger valleys created temporary glacial lakes as evidenced by extensive silt terraces along the North Thompson River valley just south of the map area. Glaciofluvial deposits in much or the area are thought to relate to deposition by meltwater during deglaciation. An exception to this is found at the confluence of the Clearwater and North Thompson rivers where thick glaciofluvial gravels exposed in sections are partially covered by a surface till. Some water well logs in the vicinity of Clearwater record thick gravel deposits beneath several metres of till. It is clear that a glacial advance occurred over pre-existing gravels. Some of these gravels may predate glaciation if the North Thompson valley was a depositional trough since the Tertiary. This is likely given that the valley is deeply incised and runs along a prominent lithological boundary.

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