Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
eastward (adv., adj.), eastwards (adv.)
Eastward is the only adjective (The window looked out on an eastward prospect), but eastward and eastwards are both Standard as adverbs (We flew eastward [eastwards] for several hours). British English tends to use only eastwards as adverb; Americans more often use eastward. Westward(s), northward(s), and southward(s) behave similarly.
