Image from Geoscape Nanaimo - Govt of Canada
The bedrock of Vancouver Island was not always located off the coast of North America. About 100 million years ago, a "terrane" of volcanic rocks called Wrangalia arrived from the Pacific and collided with the coastline creating the majority of Vancouver Island, Haida Gwaii and part of Alaska. The climate in this area was very different from today, and a warm shallow tropical sea covered much of the east coast of Vancouver Island. Between 85 and 65 and million years ago, silt, sand, and shells were deposited on this sea floor. Marine animals including large marine reptiles and ammonites lived and died in this sea. As the deposited material was covered with more deposited material, the deeper material was compressed into sedimentary rock. Calcium from sea shells formed limestone, sand formed sandstone, silt and mud formed shale and mudstone. If conditions were appropriate, the occasional animal would have its bones or shell preserved as a fossil.
Image from Geoscape Nanaimo - Govt of Canada
This sedimentary rock was deposited on top of the older volcanic rock, in some places, up to six kilometres deep. Collectively, the areas of sedimentary rock deposited during this time, are called the Nanaimo Group. The brown areas on the map show the areas where the Nanaimo Group are located.
At this location, the sedimentary rocks tell a few stories. Layers of shale and mudstone are clearly visible around the banks of the Cowichan River. On the north side of the river, where you are standing, the layers of rock come up out of the ground at an angle and are eroded such that thousands of years worth of layering is clearly visible. You'll notice that the layers are not lying flat, as they would have been when deposited on the floor of the tropical sea. Instead they have been uplifted by the huge forces created as Vancouver island was (and continues to be) to be pushed into the North American coast. On the opposite side of the river, the banks of the valley have been eroded along the face of the sedimentary sheets... you can clearly see the surface of the sheets and the steep angle at which they have been uplifted to.
If the water levels are low enough, you will be able to see the layers of exposed rock. There are two rock types layered here, mudstone and shale. The shale is made up of compressed layers of silt, and the mudstone are the thicker layers that were made by layers of fine mud.
The fast-flowing Cowichan River has been eroding this area for thousands of years, creating some beautiful shapes in the river bed and banks. Look for deep, circular pot-holes. These are eroded by a rock being swirled around in circles in a depression... eroding deeper and deeper into a circular hole. This is a high-energy part of the river. Large volumes of water flowing through narrow constrained canyons create high-erosion areas, and the tiny eroded particulate is moved downstream and deposited as sand and silt in areas where the river slows and looses energy, such as the inside of river bends and the flat Cowichan Delta area near Cowichan Bay.
In order to log this EarthCache, send us an email (through our geocaching profile) that has the answers to the following questions:
1. Looking at the shale and mudstone layers, estimate the angle (degrees from the horizontal) that the layers now lie.
2. Using your GPS, estimate the depth of erosion of the canyon at this location. You may wish to take an elevation reading at the top of the canyon, and another at the bottom of the short trail, closer to the river.
3. Optional (only if conditions allow) post a photo of you showing the layers of shale and mudstone.
The Cowichan River is a popular summer destination for tubing down the River.
The Cowichan River is an important salmon spawning ground. Fish ladders have been constructed upstream at Skutz falls to help the fish navigate the fall rapids.