Welcome to Madrona Point, a beautiful location near Parksville, British Columbia, offering a fantastic opportunity to explore Earth's history written in stone! This EarthCache will guide you in becoming a geological detective, learning to "read the rocks" exposed along the seashore and understand the fascinating story they tell. The lesson here is to differentiate types of sedimentary rock found on the central Vancouver Island beaches, which can lead you to a further and better understanding of how the rock in this area formed.
Your Task:
To log this EarthCache, please visit Madrona Point and make the following observations. Then, answer the questions below by sending me a message through Geocaching.com before logging your find.
- Find an example of sandstone. Describe its texture (gritty or smooth?) and its color. Do you see any layers within it?
- What is unique about the layers, as you look at them from the bottom, moving upwards, in the small sea stacks at the given coordinates? Why do you think this change in the layers occured? For more information on this specific geological anomaly, enjoy nearby EarthCache Cottam Point Unconformity (GC30KP9).
- Identify any nearby conglomerate rock. Describe the size and shape (rounded or angular) of the pebbles and stones within it. What type of smaller material holds these larger pieces together?
- Include a photo with your posted log of yourself or an object that identifies you (trackable, paper with your caching name, GPSr, etc) beside a sample of conglomerate rock.
Please note that access to this public beach area is via parking at the trail head (3 parking spots) or along the side of the road, per the given parking coordinates. Please be careful to not block any driveways. Also be aware that this EarthCache is best observed and completed at low tide, so check local tide tables for the best times to visit.
Geological Lesson: Reading the Rocks
The rocks you see at Madrona Point are sedimentary rocks. These rocks are formed from the accumulation and cementation of sediments – fragments of older rocks, minerals, and even the remains of living organisms. By observing these rocks closely, we can identify different types and learn about the ancient environments in which they were formed.
The primary sedimentary rock types you'll encounter here are:
- Sandstone: Feel the surface. Does it feel gritty, like sandpaper? If so, you're likely looking at sandstone. Sandstone is made from cemented sand grains and can tell us about ancient environments like beaches, river channels, or shallow marine areas where sand was deposited. Notice the layers within the sandstone, which can indicate changes in sediment type over time.
- Shale: Look for smoother rocks that tend to break into thin, flat layers. This is shale, formed from the compaction of fine mud and clay. Shale typically forms in quiet, low-energy environments like deep ocean floors or lakebeds.
- Conglomerate: These rocks are easy to spot! They look like natural concrete, with rounded pebbles and stones of various sizes cemented together. The rounded nature of the pebbles suggests they were transported by high-energy environments like fast-flowing rivers or beaches with strong waves.
The sedimentary rocks at Madrona Point belong to the Nanaimo Group. These rocks were deposited during the Late Cretaceous Period, a time when dinosaurs still roamed the Earth, approximately 90 to 65 million years ago. The sediments accumulated in a basin called the Georgia Basin.
Observations to Reinforce the Lesson:
Here are three simple observations you can make at Madrona Point to further understand the sedimentary rocks:
- Look for changes in grain size within a single rock or between different rocks. This can tell you about variations in the energy of the environment where the sediments were deposited. For example, a rock with larger grains (like conglomerate) indicates higher energy than a rock with very fine grains (like shale).
- Observe the layering in the rocks. Are the layers thick or thin? Are they all the same type of rock, or do they alternate? This layering represents different periods of sediment deposition over time.
- See if you can find any fossils within the rocks. The Nanaimo Group is known to contain marine fossils like shells. Finding a fossil provides direct evidence of the life that existed when these rocks were forming.
An example of conglomerate rock, illustrating how it is different from sandstone or shale.
Conclusion:
By making these observations at Madrona Point, you've taken a step back in time to the Late Cretaceous Period and learned to "read the rocks" like a geologist. You've identified different types of sedimentary rocks and gained an understanding of the ancient environments in which they were formed. Enjoy exploring this fascinating geological site!
Sources:
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- Conglomerate (geology) - Wikipedia, accessed April 20, 2025, https://en.wikipedia.org/wiki/Conglomerate_(geology)
- 5 Weathering, Erosion, and Sedimentary Rocks – An Introduction to Geology - OpenGeology, accessed April 20, 2025, https://opengeology.org/textbook/5-weathering-erosion-and-sedimentary-rocks
- Nanaimo Group, accessed April 20, 2025, https://web.viu.ca/earle/mal-cut/nanaimo-group.pdf
- Nanaimo Geotour 2015 - MineralsEd, accessed April 20, 2025, https://mineralsed.ca/site/assets/files/3451/nanaimo_area_geotour.pdf
- Stratigraphy, palaeogeography and evolution of the lower Nanaimo Group (Cretaceous), Georgia Basin, Canada - ResearchGate, accessed April 20, 2025, https://www.researchgate.net/publication/376076597_Stratigraphy_palaeogeography_and_evolution_of_the_lower_Nanaimo_Group_Cretaceous_Georgia_Basin_Canada
- Marine Environments of the Upper Cretaceous - Nanaimo Group, Pender Island, BC - UVIC, accessed April 20, 2025, https://www.uvic.ca/science/seos/assets/docs/PenderFT13.pdf
- Nanaimo Group - Wikipedia, accessed April 20, 2025, https://en.wikipedia.org/wiki/Nanaimo_Group
- Geology of British Columbia and Vancouver Island, accessed April 20, 2025, https://web.viu.ca/earle/geol111/geology-of-vancouver-island.pdf