Defining a Glacial Erratic
A glacial erratic is a rock or boulder that differs in size and type from the native rock of the area in which it rests. Erratics are typically large, often isolated, and were transported by glacial ice from their place of origin, sometimes over hundreds of kilometers, before being deposited when the glacier melted.
How Glaciers Transport Erratics
Glaciers, being massive sheets of ice, have immense erosional and transportational power. As a glacier moves, it picks up rocks and debris, incorporating them into its ice mass. These embedded rocks, including erratics, are then carried along with the glacier's flow. When the climate warms and the glacier retreats or melts, these transported rocks are left behind, often far from their bedrock source.
Identifying a Glacial Erratic: A Practical Example
A classic example is the 'Okotoks Erratic' in Alberta, Canada, a massive quartzite boulder weighing over 16,500 tonnes, transported from the Rocky Mountains hundreds of kilometers away. Similarly, numerous erratics are scattered across the landscapes of the American Midwest and parts of Europe, standing out dramatically against the local geology as clear evidence of past glaciation.
Significance in Geological Studies
Glacial erratics are crucial geological indicators. Their presence and composition allow scientists to map ancient glacier paths, reconstruct past ice sheet extents, and understand the dynamics of glacial movement and retreat. By studying the bedrock source of an erratic, geologists can trace the exact direction and distance of ice flow, providing vital clues about Earth's climatic history.