BC's Oldest Seafloor EarthCache
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This cache was placed for the BCGA Cache Blitz '13, and won the award for Best Educational Spot/Environmental Awareness for Kootenay/Rockies
Welcome to Marysville Falls, where you can view BC's oldest seafloor.
The questions here will be easiest to answer during low water, but you can easily see enough rock to answer them at any time of year.
Please DO NOT cross the fence. It is there for your safety, and the local Search and Rescue team prefers to only ever need to come here for practice.
Most of the southern Purcell Mountains, including the rock exposed here, are sandstone and mudstone. Geologists estimate that this rock is over 1.4 billion years old. At this time, a giant continent was splitting, and an inland sea filled the valley. Layers of sand and mud were deposited, becoming sandstone and mudstone over time and under pressure. Ground waters added minerals that helped to glue the grains together. Hundreds of millions of years later, the broken continent split for good and the western portion, now its own tectonic plate, floated away on the currents of the Earth's mantle. Geologists suspect that the departed plate is now part of either Australia or Siberia. Back in what was to become the East Kootenays, the eastern half of the broken continent remained and became North America, with where you're standing as its western edge.
As the Atlantic Ocean opened up and the North American plate drifted west, the edge of the plate (where you're standing) began to collide with ocean floor and volcanic islands. The ancient rocks of the seafloor were broken and folded during the tectonic collisions, creating the Purcell and Rocky Mountains. The islands and ocean floor were welded onto North America, and form most of British Columbia, west of Kootenay Lake.
Here, Mark Creek has carved a canyon through the rock of the ancient seafloor, allowing us to see the striped layers of sandstone and mudstone that were deposited billions of years ago. The mud was carried by rivers and slowly settled to the seafloor. Sand, also carried by rivers, settled closer to shore and occasionally collapsed in undersea landslides, creating a layer of sand. The image below is a schematic of the deposition.
To log this Earthcache, email the owner the answers to the following questions:
1) What colours are the layers of rock? Which layer (sandstone/mudstone) is darker?
2) What do you think might cause the reddish colour that you can see in places?
3) Please comment on the position of the falls relative to the St. Marys River and whether it's changing (hint: Mark Creek dumps into the river a short distance downstream)
Sources:
Turner, R.J.W., Anderson, R.G., Franklin, R., Grieve, D., GeoTour Guide for the East Kootenay, British Columbia, Geology, Landscapes, Mines, Ghost Towns, Gold, Dinosaur Tracks, and Hot Springs, Geological Survey of Canada Open File 6477/British Columbia Geological Survey Geofile 2010-07.
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