Here's why:

* Glacial erratics are large rocks or boulders that have been transported by glaciers and deposited in a location far from their original source. This is why they're resting on a different type of bedrock.

* Glacial zones are areas where glaciers form and move. This is where erratics are most commonly found.

How it works:

1. Glaciers form: Ice accumulates and compresses, forming a massive sheet of ice.

2. Glaciers move: The weight of the ice causes it to flow slowly downhill, carving out valleys and eroding the land.

3. Erosion and Transportation: Glaciers pick up rocks and boulders as they move, transporting them along with the ice.

4. Deposition: As the glacier melts, it deposits the rocks and boulders it carried, often far from their original location.

The difference in bedrock type is a key indicator that the boulder was transported by a glacier.