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 ancient clay that is exposed along the banks of the Grand River between Onondaga and Caledonia. This clay was deposited by proglacial lakes during the nlast ice age, and has been here for more than 12,000 years! This earthcache will review the proglacial lakes that deposited the clay in the area, and what other evidence of these ancient lakes is visible from this location.
Please note: You will need a boat of some sort to access GZ, as walking in from thr roas 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 Sediment Layers of the exposed bank at this location. What colours do you see in the layers? Is the material fine (clay/silt) or coarser (sand/gravel)? Do the layers appear sharp and distinct, or blended/mixed? Using your senses, do you think that this is clay, sand, silt or another material?
2. What’s on Top of the clay? A dark organic-rich topsoil? Loamy or sandy material? More clay? What does this layering suggest about the more recent geological history of the site?
3. Elevation Estimation: Use a GPS-enabled device to estimate your elevation above sea level. What does this elevation tell you about which proglacial lake(s) might have once covered this particular location?
4. Geological Interpretation: Based on the clay layers, elevation, and what lies above them, what can you infer about the depositional history of this site? Explain how the evidence supports your conclusion.
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:
Standing along the Grand River between Onondaga and Chiefswood Road, you are gazing upon the silent evidence of vast, ancient lakes that once covered this part of Southern Ontario. More than 12,000 years ago, this quiet river valley was submerged beneath the cold, sediment-rich waters of two immense proglacial lakes: Glacial Lake Whittlesey and later, Glacial Lake Warren. These lakes were not isolated events but part of a broader, dynamic sequence of glacial retreat and meltwater pooling that shaped the entire region’s topography during the final stages of the last Ice Age.
As the Laurentide Ice Sheet began to melt and retreat northward, it released colossal volumes of meltwater into the Great Lakes basin. With no open northern drainage due to the lingering ice, water pooled against the ice front and the surrounding highlands. The result was the formation of enormous proglacial lakes, which inundated vast swaths of land—including modern-day Brant County. Between approximately 14,500 and 12,000 years ago, the area where you now stand lay beneath these glacial lakes for centuries at a time.
Glacial Lake Whittlesey formed first, around 14,000 years ago, with a surface elevation estimated between 230 and 240 metres above sea level. This lake completely submerged the lowlands surrounding Brantford and Onondaga, depositing fine sediments across the lakebed in slow, quiet layers. These calm waters allowed the finest materials—microscopic particles known as glacial rock flour—to settle out of suspension. Over time, this process created thick beds of bluish-grey clay, now visible in riverbanks and road cuts throughout the region.
When Lake Whittlesey eventually drained through lower outlets as the ice sheet retreated, a slightly lower lake—Glacial Lake Warren—took its place, occupying the same general footprint. This new lake had a surface elevation of approximately 205 to 215 metres above sea level. Like its predecessor, Lake Warren persisted for many centuries, continuing the quiet work of clay and silt deposition. These fine-grained lake-bottom sediments still shape the land today, influencing soil types, drainage patterns, and even modern agriculture.
The clay you see exposed in the riverbanks near this EarthCache site is a direct remnant of these long-vanished lakes. These layers formed when glacial meltwater, heavily loaded with sediment, flowed into the still waters of Lake Warren. As the energy of the water diminished, sediments settled in order of size: coarse gravel and sand dropped out quickly near inflows, while silt and, finally, clay settled in deeper, more distant parts of the lake. The clay found here likely accumulated in the quietest parts of the lakebed, forming a dense, sticky, and nearly impermeable layer that still resists erosion today.
Glacial rivers carried this suspended sediment into the lakes. As the water slowed upon entering the still lake basin, the sediment began to settle.
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Heavier sand and gravel dropped out first, closer to the glacial inflows.
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Finer silts fell out more gradually, forming thin laminations.
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Clay, the finest material of all, remained suspended for the longest—eventually settling in the deepest and calmest waters as smooth, even layers
This lakebed clay is typically smooth to the touch when wet, dries to a hard, cracked surface, and can appear grey, bluish-grey, or even tan when oxidized. In vertical bank exposures, you may notice that it lacks the layering seen in silt or sand—its texture is more massive, homogeneous, and cohesive. This is a defining trait of clay deposited in low-energy environments, where only the finest particles have the time and stillness to settle.
After the proglacial lakes drained—either catastrophically or gradually—this area transformed. Over top of the clay, later layers may be present, reflecting changes in the environment after the lakes drained. These might include a veneer of silty floodplain deposits, sandy outwash from meltwater channels, or darker organic soils from more recent wetland or riparian development. The result is a vertical geological storybook: older glacial lake sediments below, capped by more recent postglacial and modern deposits above
This creates a distinct vertical profile:
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Top layer: dark, organic-rich soil or silty floodplain loam
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Middle layer: coarser material or interbedded sands/silts (if present)
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Bottom layer: massive, bluish-gray lake clay
In many locations along the Grand River near Onondaga, this full sequence is visible in bank exposures, especially during low water conditions or after bank collapse.
Field Clues: How to Recognize Ancient Lakebeds
Standing here today, you can spot signs of the lakes’ former presence:
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Flat terraces: Formed as former lake shorelines, now farm fields or gentle slopes
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Fine-grained sediments: Exposed in vertical banks or erosion scars
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Cracked clay: Hard and dry in summer, wet and sticky after rain
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Layered sequences: Sand or silt capping dense clay
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Elevations below 215 m: A good indicator of areas once submerged by Lake Warren
The river’s natural erosion has revealed the clay beds in situ, giving us a rare window into the ancient lake floor beneath our feet.
References
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Chapman, L.J., & Putnam, D.F. (1984). The Physiography of Southern Ontario. Ontario Geological Survey.
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Karrow, P.F. (1987). Quaternary Geology of the Hamilton-Cambridge Area.
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Ontario Geological Survey. Surficial Geology Mapping.
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Eyles, N. (2002). Ontario Rocks: Three Billion Years of Environmental Change.
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Geological Survey of Canada. Late Quaternary Sediments and Glacial History of Ontario.