Welcome to my Earthcache! An Earthcache is a special type of geocache where there is no container to find - instead you are looking for a unique geological feature of the area and need to answer questions, as well as posting a picture, in order to claim the find. At this location, you will be observing the erosion of the blue-grey clay that was deposited here by the Proglacial Lake Warren approximately 12,000 years ago, and how it erodes differently than the surrounding soil and sand.
Please note: You will need a boat of some sort to access GZ, as walking in from the road would be dangerous, and result in trespassing. You will need to put in at one of the waypointed boat launches, and paddle (motorized vessels are also permitted here) to the coordinates to make your observations. Please make sure you are staying safe while doing this - bring all of the legally required safety gear with you, and make sure you paddle according to weather and water conditions. It will not be possible to complete this earthcache in the winter or during the spring floods.
As with all of my ECs, I am not looking for PhD thesis level responses, but I am hoping that you take some time to enjoy the area and learn something new. Please include a list of all cachers with your answer, if answering for more than one caching name.
To claim a 'find' for this Earthcache you must answer the following questions and send your answers in a message or email to the owner using the link at the top of the page. You can log your find with a photo at GZ. Send your answers to the tasks. I will be in contact if there is a problem, no need to wait for a response as long as the required photo is included in your log.
Questions to Answer:
1. Describe the exposed clay in this location. What colour is it? What is the texture (smooth, gritty, crumbly)? Does it appear to be layered or one massive block?
2. Observe erosion patterns. Does the bank appear to be eroding in horizontal sheets or vertical chunks? Are there visible cracks or separations? What does this tell you about the clay’s properties?
3. Compare with surrounding soils. Do you see any looser soils or silt above or below the clay layer? How do they differ in appearance and erosion style?
4. Interpret the erosion process. Based on what you observe, explain why the clay bank is eroding differently than the surrounding material. What does this tell you about the composition and structure of the clay?
5. Photo Requirement: Take a photo of yourself (or just your GPS device, or your geocaching name on a piece of paper, or your signature item, a thumbs up) with the clay banks in the background. You are not required to show your face in the photo, but each geocacher should upload an image with their log indicating that they were there. You are also welcome to share other photos of your visit to this beautiful area, but at least once image should show you with the clay visible in the background.
Earthcache lesson:
In this stretch of the Grand River near Onondaga, exposed clay banks are visible year-round. These clays were deposited during the retreat of the Laurentide Ice Sheet between 14,000 and 12,000 years ago, when this region was submerged under Glacial Lake Warren. Over centuries, fine glacial flour settled out of the still waters and created thick beds of laminated clay, now visible in cross-section where the river has cut into them.
You may also see more modern deposits on top of the clay, such as silt from seasonal floods or darker soils from past wetlands. These materials erode differently than the ancient clay beneath them—often faster and more irregularly—allowing you to compare natural erosion processes in real time.
Unlike loamy or silty banks, which crumble or undercut easily, clay banks tend to erode in distinctive horizontal layers, resembling stacked bricks. This EarthCache explores why clay behaves this way, what makes it different from surrounding soils, and how its unique properties influence erosion and riverbank stability.
What Is Clay, Really?
Clay is not just "really fine dirt"—it's a specific type of material made up of microscopic mineral particles that are usually less than 0.002 mm in diameter. These particles are so small that individual grains can’t be seen with the naked eye. But clay’s behavior is not just about size—it's also about mineral structure and water chemistry.
Most clays are composed of minerals like kaolinite, illite, or smectite, all of which have plate-like or sheet-like crystal structures. These minerals are formed from the chemical weathering of feldspar-rich rocks such as granite and basalt. The layers of these sheets hold onto water and ions between them, making clays pliable when wet and hard when dry.
Because clay particles are flat and tightly packed, they have low permeability (meaning water doesn't easily flow through them) and high cohesion (meaning the particles stick together well). This makes clay banks dense, slippery, and strong—but only under certain conditions.
Why Clay Banks Erode Like Bricks
Erosion is the process of materials being worn away by natural forces like water, ice, and wind. In silty or sandy soils, water easily penetrates the ground, softens it, and causes slumping or undercutting, where chunks fall away irregularly. But clay behaves differently.
Here's why clay banks erode in horizontal sheets or slabs:
1. Layered Deposition - Clay in Southern Ontario was often deposited in ancient glacial lakes, such as Glacial Lake Warren. These lakes had calm, still waters that allowed clay to settle in thin, uniform layers. Over time, these accumulated like pages in a book—one season's sediment atop the last—creating stratified beds.
2. Weak Planes Between Layers - Although the clay itself is cohesive, the boundaries between each depositional layer are natural planes of weakness. When water seeps into these boundaries or freezes and thaws, it can break the bank along these planes. This is why erosion often happens in slabs or plates, resembling bricks or shale.
3. Low Permeability = Surface Saturation - Because clay is nearly impermeable, rainwater or river splash doesn’t soak in. Instead, it collects on the surface or trickles along tiny cracks. When the clay becomes saturated, it swells and loses strength, eventually causing a thin layer to shear off and slide away. Unlike sandy soils, clay fails in thin sheets, not clumps.
4. Drying and Cracking - When exposed clay dries out, it shrinks and forms polygonal cracks on the surface. These cracks deepen with repeated wet-dry cycles, which promotes separation between layers. Later rains or river flow can exploit these cracks, causing pieces to detach horizontally rather than vertically.
5. River Undercutting is Rare - Unlike looser soils that collapse from below when water removes support, dense clay banks often remain intact underneath for long periods, resisting undercutting. Instead, erosion occurs from the top down or across surface layers, contributing to the “brick wall” appearance.
References
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Chapman, L.J., & Putnam, D.F. (1984). The Physiography of Southern Ontario. Ontario Geological Survey.
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Eyles, N. (2002). Ontario Rocks: Three Billion Years of Environmental Change.
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Karrow, P.F. (1987). Quaternary Geology of the Hamilton–Cambridge Area.
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Ontario Ministry of Natural Resources and Forestry – Surficial Geology Mapping.
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Goudie, A. (2006). The Nature of the Environment. Oxford University Press.