Where did Moses Get the Information to Write the firs 11 chapters of Genesis?

How did Moses get the information to write the history of the first 11 chapters of Genesis? Divine revelation on the mountain top is most likely BUT consider this. According to the genealogy of Gen 11, Shem, the son of Noah, was living at the time of Abraham. Noah's father, Lamech, was a young man before Adam died! Oral and written records of conversations of Adam and what life was like before the Fall could have been easily passed down. Here's a scenario. ADAM talks with METHUSELAH who talks with SHEM who talks with ABRAHAM. Abraham could have had a first-person account of the FLOOD by talking with SHEM!