This post by Mark Nichol originally appeared on Daily Writing Tips.
Determining whether to refer to geological and astronomical terms with initial uppercase or lowercase letters can be a challenge, because various publications and publishers differ on capitalization style. The following guidelines, however, appear to predominate:
Names of geological time spans are capitalized, but the terms for the magnitude of duration (eons, eras, periods, epochs, and stages, in descending order of length), are not; in scientific and nonscientific prose alike, these terms can be omitted:
“The Mesozoic is also known as the Age of Dinosaurs.”
“Mrs. Wattle has been teaching Freshman Composition since the Mesozoic.”
Whether modifying terms such as early, middle, and late are capitalized depends on whether they are themselves modified:
“Tyrannosaurus rex lived during the Late Cretaceous.”
“The Deccan Traps erupted in the very late Cretaceous.”
“Ice age” is considered a generic term because multiple such events have occurred.