Mike,
American Heritage Dictionary defines a sierra is "a rugged range of
mountains having an irregular or jagged profile".
I did a quick web search and found sierras being used quite often.
Here are some old sources:

Book: In the Heart of the Sierras by James M. Hutchings (1888)
Movies: Springtime in the Sierras (1947); King of the Sierras (1938)

So if there are more than one mountan ranges, it appears that the
plural term sierras is used such as Sierras de Cazorla, Segura and Las
Villas in Spain

http://www.andalucia.com/environment...ct/cazorla.htm