This rock was left here by the Wisconsin Glacier 13,000 years ago. Notice that the rock has been split down the middle. This was caused by a type of mechanical weathering called frost wedging.
The crack grew by the constant freezing and thawing of water. At first this split was a small crack. After it would rain, the crack would fill and hold water. Then the water would freeze and expand to make the crack grow bigger. This process has happened over and over again causing the rock to look the way it does now.