grammatical number - Should decades (e.g. '70s) be singular or plural

Are specific decades singular or plural? Which of the following is grammatically correct?

The '70s was a great period for rock music.

or

The '70s were a great period for rock music.

Please provide references if you have any.

Answer

My interpretation would be this (including source).

Rule 8. Use a singular verb with distances, periods of time, sums of money, etc., when considered as a unit.

The '70s is a period of time

Examples:
Three miles is too far to walk.
Five years is the maximum sentence for that offense.
Ten dollars is a high price to pay. BUT Ten dollars (i.e., dollar bills) were scattered on the floor.

Rule 8: Blue book of grammar and punctuation