Steno observed what?

Steno, or Nicolaus Steno, was a 17th-century Danish scientist who is often regarded as one of the founders of geology and Paleontology. He observed the principles of stratigraphy, including the law of superposition, which states that in undisturbed sedimentary rock layers, older layers are found beneath younger ones. Steno also recognized the importance of fossils, proposing that they were the remains of once-living organisms, which contributed significantly to the understanding of Earth's history and the evolution of life. His work laid the groundwork for modern geological and paleontological sciences.