Mill Creek is one of the many small tributaries to Okanagan Lake. It contributes about 5% of the water which enters the lake each year. In comparison, Mission Creek, the largest tributary, contributes approximately 25-30% of the water which flows into Okanagan Lake each year. Mill Creek originates at Postill Lake, and flows approximately 31kms before it reaches Okanagan Lake just south of the bridge. The creek has cut a beautiful rugged valley through the bedrock as it descends from the highlands, and then formed an alluvial fan as it reached the flatter valley floor. Much of Ellison is built on this alluvial fan deposited by Mill Creek, which was a much larger creek 8,000 to 10,000 years ago.
The basement rocks in the Okanagan Valley, as well as the highlands surrounding the valley, are igneous in nature. Dilworth Mountain, Knox Mountain, Layer Cake Hill, and Black Knight Mountain formed from local volcanic activity approximately 50 million years ago (mya). More recent volcanic activity, including the King Edward Formation, approximately 11 to 20 mya, formed deposits in the upland area between Mission Creek and Vernon.
Granite and basalt are two different igneous rocks. They both form as rock liquefies when pressure above is suddenly released, approximately 65km below the earth’s surface, causing the magma to move up toward the surface, cool, and solidify. If the rock cools well below the surface it does so slowly, allowing coarse crystals to grow. These crystals give the rock a salt-and-pepper appearance. If, on the other hand, the magma reaches the earth’s surface, it cools much more quickly, leading to a dense, gritty rock. The coarse-crystalized rock is granitic; the dense, gritty rock is basaltic.
Granitic rocks can undergo metamorphosis caused by very high temperature and pressure. This metamorphic rock is called gneiss, and is usually banded grey and white.
References:
Cannings, S., Nelson, J., and Cannings, R. 2011.Geology of British Columbia: A Journey through Time. Greystone Books, Vancouver, BC.
Rae, Rowena. 2005. The State of Fish and Fish Habitat in the Okanagan and Similkameen Basins. Prepared for the Canadian Okanagan Basin Technical Working Group, Westbank, BC.
Roed, M.A., and Greenough, J.D., eds. 2004. Okanagan Geology British Columbia, Kelowna Geology Committtee.
Swain, L.G. 1990. Water Quality Assessment and Objectives: Okanagan Area Tributaries to Okanagan Lake Near Kelowna. Ministry of Environment, Province of British Columbia.
To log this earthcache, please email the cache owner with your answers to the following questions:
1. What is the approximate width of Mill Creek at the edge of the falls?
2. Based on their appearance, are the rocks found at the falls granitic , gneissic, or basaltic? Why do you think so?