Plucked by Ice EarthCache

🧊 Plucked by Ice: Reading Evidence in a Glacial Boulder  
Southern Vancouver Island – Fraser Glaciation

🌎 Geological Setting

During the Late Pleistocene, southern Vancouver Island was covered by the Cordilleran Ice Sheet. The most recent major glaciation in this region, the Fraser Glaciation (~25,000–11,000 years before present), buried the landscape beneath thick moving ice.

In the Victoria and Saanich Peninsula region, ice flowed predominantly south to southeast toward the present-day Strait of Juan de Fuca.

Glaciers reshape landscapes through:

• Abrasion – grinding and smoothing bedrock  
• Plucking – removal of rock blocks along fractures and joints  

This EarthCache focuses specifically on joint-controlled glacial plucking and how pre-existing fractures influence the size and shape of transported blocks.

🪨 Site Description – Field Observations

The boulder at this site is large, isolated, and rests on soil with no visible connection to surrounding bedrock.

Field measurements recorded on site:

• Length ≈ 3.0 m  
• Width ≈ 2.2 m  
• Height ≈ 1.6 m  
• Circumference at thickest section ≈ 8.6 m  

Using an ellipsoid approximation, the estimated volume is approximately 5–6 cubic metres.

Assuming a density typical of granitic rock (~2.65 tonnes per cubic metre), the estimated weight is approximately 14–16 tonnes.

This represents a substantial mass of rock transported by glacial ice.

🔬 Joint-Controlled Plucking

A prominent fracture (joint) can be traced along part of the boulder’s surface before disappearing beneath moss.

Joints are natural fractures that form due to:

• Cooling contraction in igneous rocks  
• Tectonic stress  
• Pressure release during uplift and erosion  

Glaciers preferentially exploit these planes of weakness. When ice freezes into cracks and applies stress, it can detach entire blocks along joint planes. This process is called plucking.

The visible fracture on this boulder likely represents a pre-existing structural weakness that influenced how the block was detached from its source outcrop.

After deposition, freeze–thaw weathering may have widened the fracture further.

🧊 Transport and Deposition

Regional south–southeast ice flow during the Fraser Glaciation is well documented across southern Vancouver Island.

A block detached along joint planes north of this location could have been transported within or beneath the glacier and deposited here as the ice retreated.

Its isolation and lack of connection to local bedrock support interpretation as a glacial erratic deposited during deglaciation more than 10,000 years ago.

This site provides an opportunity to interpret:

• How joint orientation influences block removal  
• How glacial movement transports large masses  
• How erratics relate to regional ice-flow direction  

📋 Logging Requirements

To log this EarthCache, please MESSAGE me the answers to the following - with the Earthcache title please.

1. Examine the visible fracture. Does it appear to represent a pre-existing joint that could have been exploited by glacial plucking? Describe one observable feature that supports your interpretation.

2. Compare this boulder to nearby rock fragments or exposed bedrock in the area. Does it appear similar or different? How does this comparison support or challenge the interpretation that it is a glacial erratic?

3. Regional ice flow in this area was south to southeast. Explain how this direction of movement supports the interpretation that this block was transported from a different source location.

4. Why are glaciers more likely to remove blocks along joint planes rather than from solid, unfractured rock?

📸 Photo Requirement:

Please post a photo of yourself or a personal item with the boulder visible.

Please do not post close-up images of the fracture or any photos that reveal answers to the questions.