Surficial geology, Lac La Hache, British Columbia

Abstract

The Lac la Hache map area (92P/14) dominantly lies within the Fraser Plateau physiographic region except for the valley of Lac la Hache which is part of the Fraser Basin. The Fraser Plateau is a region of rolling topography with an average elevation of approximately 1200 m above sea level and the Fraser Basin is the dissected portion of the plateau. The Lac la Hache map area includes an important drainage divide. A large part of the map area drains to the west towards Lac la Hache which empties into San Jose River, a tributary of Fraser River. The eastern sector of the map area drains to the east and is part of the North Thompson River drainage. The region is predominantly underlain by till which consists of a poorly sorted diamicton with clasts of all size. Bedrock outcrops are rare and limited in extent. In some of the valleys and the low areas, till is overlain by glaciofluvial sediments which were deposited in meltwater streams at the end of the last glaciation. Glaciolacustrine sediments, deposited in glacial lakes which formed at the end of the last glaciation, were mapped in the Bridge Creek, Lac la Hache, and the upper Bradley Creek valleys. Lacustrine sediments are present at the periphery of modern lakes throughout the map area. These were deposited at a time when the lakes occupied a higher level. Modern stream sediments (alluvium) are present in extent large enough to be mapped in some of the valleys. Colluvial deposits are mapped on steep slopes throughout the map area. One large landslide was mapped approximately 5 km south of Spring Lake. During the last glaciation, probably at glacial maximum, ice was dominantly moving to the south southwest to southeast as recorded by flutings, crag-and-tails, drumlins, and striations. However, an earlier phase of ice flow generally to the west is recorded by flutings southwest of Lang Lake, southeast of Rail Lake, and north of Timothy Lake. This first ice-movement is also recorded in the glacial striation record. During deglaciation, both the eastward and westward drainages were blocked by ice and/or sediments which lead to the development of glacial lakes in the valleys. A large partly buried valley, probably related to a Tertiary drainage system, extends southeast from Lac la Hache. This valley was in part eroded by glacial meltwater and hence is mapped as a large meltwater channel.