Where Erratics are Born EarthCache

A glacial erratic is glacially deposited rock differing from the size and type of rock native to the area in which it rests. "Erratics" are carried by glacial ice, often over distances of hundreds of kilometres.

But where does an erratic come from? At the posted coordinates, you will find an example of how glacial erosion can form potential erratics.

Glaciers erode rock by multiple processes: abrasion/scouring, ice thrusting, spalling, and plucking. In an abrasion process, debris in the ice scrapes and gouges the underlying rocks like sandpaper, producing smaller glacial till. In ice thrusting, the glacier freezes to its bed, moving large sheets of frozen sediment into mounds. Spalling occurs when ice lens formation with the rocks below the glacier cause surface layers of rock to flake off into small pieces. In the process of plucking, glaciers crack chunks of bedrock off, producing larger particles.

Plucking is responsible for the erosion and transportation of individual pieces of bedrock, especially large "joint blocks". Friction and pressure cause the ice at the bottom of the glacier to melt and infiltrate cracks in the bedrock. This freezing and thawing action causes the cracks to expand, gradually loosening the rock and producing large pieces of rock called joint blocks. Eventually these blocks come loose and become trapped in the glacier.

Rocks of all sizes can become trapped in the bottom of the glacier. Glacial plucking is most significant where the rock surface is fractured or where bedrock is exposed, as this allows meltwater to penetrate more easily. As large chunks break off, they exert additional stress on the bedrock, contributing to further erosion.