An isolated boulder left by a glacier is called an erratic.

Here's why:

* Glaciers are powerful forces of erosion and transportation. They can pick up and carry massive rocks, often far from their original location.

* As glaciers melt, they deposit the rocks they've been carrying. These rocks, often of a different type of rock than the surrounding area, are called erratics because they are "errant" or "out of place."

So, next time you see a large, out-of-place boulder, remember it might be an erratic, left behind by a glacier long ago!