Puzzler

The correct answer to last week's puzzler was “Gunter's Rule”. You can see one from my collection here.

For this week's question, we turn to natural history. Nearly the entire country of Estonia has a similar geological makeup: a thick layer of limestone covered by a thin layer of soil (which is chock full of limestone rocks). However, almost anywhere in Estonia you can find rocks (some as large as a house) made of a pinkish granite. These rocks are called “erratic boulders” by geologists, and they are more common in Estonia than anywhere else in the world. How did the erratic boulders get there?

As always, no fair googling…