Grammar: southwards or southward?
The form southward is strictly speaking an adjective, such as "southward migration" and southwards is an adverb, so used in constructions such as "the troops continued southwards".
Think of: southward migration (adjective).
Think of: the troops continued southwards (adverb).
About the author
Jesse Karjalainen is the author of The Joy of English: 100 conversations about the English Language, Cannibal – the language and history of the discovery of the New World, and Roanoke – the language and history of Early Colonial America.