Tolkien's imagined prehistory of the Earth, where in ages past Elves, Dwarves and Ents lived. With each successive age, more of the original created perfection falls and fades. By the end of the Third Age and the War of the Ring, the Elves, Dwarves, Ents and other races are in the last stages of decline. With the destruction of the Ring, Sauron's evil is banished, and his Nazgul, Trolls, and Orcs are destroyed or scattered. In the Fourth and later ages, the world takes on its present composition and peoples. See for instance Kocher, Paul H. Master of Middle-Earth: The Achievement of J.R.R. Tolkien, 1974, Penguin Books, ISBN 0140038779, pages 8-15.

Tolkien discusses the Sixth Age in Letter 211 to Rhona Beare, 14 October 1958, last footnote, where he writes:

"I imagine the gap [since the War of the Ring and the end of the Third Age] to be about 6000 years; that is we are now at the end of the Fifth Age if the Ages were of about the same length as Second Age and Third Age. But they have, I think, quickened; and I imagine we are actually at the end of the Sixth Age, or in the Seventh."

Carpenter, Humphrey, ed. (1981), The Letters of J. R. R. Tolkien, Boston: Houghton Mifflin, ISBN 978-0-395-31555-2.

Shapes of continents are purely schematic.