One of our best primary sources for information about the Gorgons comes from the Theogony of Hesiod. In this poem that describes the birth of the Greek gods and goddesses, we learn that the Gorgons were the daughters of Phorcys and Ceto. In addition, we find some other important hints about the nature of these monsters from Hesiod:
"To Phorcys Ceto bore the fair-cheeked Graiae...
then the Gorgons, who dwell beyond glorious Okeanos,
at earth's end, toward night, by the clear-voiced Hesperides,
Stheno, Euryale, and ill-fated Medusa
who was mortal; the other two were ageless and immortal."
(Hesiod, Theogony, 270-277)
So from this passage in the Theogony, we know that the Gorgons were the sisters of the Graiae; that they lived in the far West; that they were named Stheno, Euryale, and Medusa; and that two of the sisters - Stheno and Euryale - were immortal, while the third - Medusa - was mortal and therefore could be killed (and indeed, Medusa, as the sole mortal sister, is most famous in mythology for the story of her tragic life and death).
As was previously mentioned, the Gorgons are also notorious for their distinctive appearance. According to ancient Greek artists, these monsters were not just ugly - they were hideous. The Gorgons are commonly portrayed in ancient art with snakes for hair, enormous tongues, teeth that would make a boar blush with envy, and a stare that could literally petrify. However, it is also worth noting that in some versions of the myth of the Gorgons, Medusa is described as a beautiful woman who makes the unfortunate mistake of comparing her beauty to that of the goddess Athena - see the page about Medusa for more details.
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Who's Who in Classical Mythology
This book is a great source for information about Greek and Roman mythology! Organized alphabetically, this who's who features information about over 1200 of the most intriguing characters - and creatures - from Classical myth and legend. And the book features a great description of the Gorgons.