What is an erratic boulder?
An erratic boulder is a large rock that has been transported a significant distance from its original source, typically by glaciers.
Best evidence for an erratic boulder:
The best evidence to determine if a boulder is an erratic is its location and composition.
* Location: Erratic boulders are often found in areas where the bedrock geology is different from the boulder's composition. For example, a granite boulder sitting on top of limestone bedrock is a strong indicator of an erratic.
* Composition: The rock type of the boulder should be different from the surrounding bedrock. This points to it being transported from another location.
* Shape: Erratic boulders often have a distinctive shape. They may be rounded, angular, or striated (scratched) from their journey within the glacier.
* Glacial features: The presence of other glacial features like glacial till (unsorted sediment), striations on bedrock, or drumlins (elongated hills) in the same area can provide further evidence that the boulder was transported by a glacier.
Example of a strong statement:
"The granite boulder, located on top of limestone bedrock, is likely an erratic because its composition differs significantly from the surrounding rock, suggesting it was transported from a distant location."
Important Note: Just finding a large boulder doesn't automatically make it an erratic. You need to consider the geological context and its potential origin to make a definitive determination.
