Complex sentence: He received his Ph.D. in geology and geophysics from Yale in 1983, and is best known for his argument that the Great Sphinx of Giza is much older than conventionally thought and that possibly some kind of catastrophe has wiped out other evidence of a significantly older civilization.
Split into simpler sentences: He received his Ph.D. in geology and geophysics from Yale in 1983. He is best known for his argument that the Great Sphinx of Giza is much older than conventionally thought and that possibly some kind of catastrophe was responsible for wiping out evidence of a significantly older civilization.