Banff National Park - Geology

Geology

The Canadian Rockies consist of several northwest-southeast trending ranges. Closely following the continental divide, the Main Ranges form the backbone of the Canadian Rockies. The Front Ranges are located east of the Main Ranges. Banff National Park extends eastward from the continental divide and includes the eastern slope of the Main Ranges and much of the Front Ranges. The latter include the mountains around the Banff townsite. The foothills are located to the east of the Park, between Calgary and Canmore. On the other side of the Park, the Western Ranges pass through Yoho and Kootenay National Parks. Still farther west is the Rocky Mountain Trench, the western boundary of the Canadian Rockies region in British Columbia.

The Canadian Rockies are composed of sedimentary rock, including shale, sandstone, limestone and quartzite, that originated as deposits on a continental shelf, similar to the relatively shallow shelf off-shore of the eastern US. The geologic formations in Banff range in age from Precambrian eon to the Jurassic period. Rocks as young as late Cretaceous, formed from the shedding of sediments towards the continental interior from rising mountain ranges farther west, were themselves caught up in the mountain-building as deformation progressed inland. However, most of these deposits—notably the Cretaceous sandstones exposed in the footwall of the McConnell thrust fault at Yamnuska mountain—are outside of the boundaries of Banff park proper.

Although the rocks in Banff Park were laid down as sediments between ~600 Ma to ~175 Ma, the main period of mountain building occurred between 80–120 million years ago, as a result of the shortening and deformation of the ancient continental shelf as exotic island terranes collided and were accreted onto the continent's margin. The shortening was accommodated by thrust faults and associated folds.

Erosion accompanied and outlasted the uplift of the Canadian Rockies, with a probable rejuvenation and acceleration of erosion rates since Pliocene time, as the Earth entered a period of extensive glaciation. Glacial landforms overwhelmingly dominate Banff's geomorphology, with examples of all classic glacial forms: cirques, aretes, hanging valleys, moraines, U-shaped valleys et al. The pre-existing structure left over from mountain-building has strongly guided glacial erosion: mountains in Banff include complex, irregular, anticlinal, synclinal, castellate, dogtooth, and sawback mountains and many of the mountain ranges trend North-Northeast, with sedimentary layering dipping down to the West at 40 - 60 degrees. This leads to dipslope landforms, with generally steeper East and North faces, and trellis drainage, where rivers and old glacial valleys followed the weaker layers in the geological succession.

Classic examples are found at the Banff townsite proper: Mount Rundle is a classic dip slope mountain, and the Spray and Sulphur river drainages flow parallel to the geological strike of the mountain range. Just to the North of Banff townsite, Castle Mountain exemplifies a castellate shape, with steep slopes and cliffs. Castle Mountain is composed of Cambrian rocks of the Cathedral formation (limestone), the Stephen shale above it, and the Eldon formation (limestone). Dogtooth mountains, such as Mount Louis, exhibit sharp, jagged slopes. The Sawback Range, which consists of near-vertically dipping sedimentary layers, has been eroded by cross gullies. Scree deposits are common toward the bottom of many mountains and cliffs.