Born on January 11, 1638, Steno was a Danish bishop and scientist best known for his "principle of original horizontality," stating that layers of sediment are originally deposited horizontally due to gravity, as well as "law of superposition," which states that layers of sediment are deposited in such a way that the oldest reside on the bottom and the youngest on top.
With his research, Steno laid down the foundations of stratigraphy, a branch of geology that studies rock layers. He also had notable contributions to paleontology, as he was one of the first to suggest that fossils were remains of once-living organisms.
Steno died in 1686. He converted to Catholicism from Lutheranism, and Pope John Paul II beatified him in 1988.
Bonus: Here are the best Google Doodles of 2011: