The rock is granite. Granite forms as large bodies of magma (molten rock) that cools while it is still underground. These blobs of granite are often more than 50 miles across. Since the granite cools underground, it forms under pressure. When it is uncovered, it expands slightly, causing the rock to crack into chunks. The points and edges weather faster, forming round boulders of granite that litter the countryside. Sometimes these can be quite spectacular, such as Elephant Rocks State Park in Missouri. I visited Elephant Rocks as a child, and still have vivid memories of exploring the huge, rounded boulders of granite.