A stromato-what?

In 2008, an approximately 500 million-year-old stromatolite was discovered at the Boxley Blue Ridge Quarry near Roanoke, Virginia. Fragments and sections of stromatolites are fairly common, but it is very rare for a whole stromatolite head to be collected intact, especially one as well preserved as the Boxley stromatolite. The fact that it is over 5 feet in diameter and weighs over 2 tons makes its intact status all the more remarkable. What is this fossil you ask? A stromatolite is a mound produced in shallow water by mats of algae that trap mud and sand particles. Another mat grows on the trapped sediment layer and this traps another layer of sediment, growing gradually over time. Stromatolites can grow to heights of a meter or more. They are uncommon today but their fossils are among the earliest evidence for living things.