You say Sierras, I say Phooey!
GG has spent a lot of time (a really really lot of time, actually) both in and studying the Sierra Nevada of California. A few years back, in being interviewed for a newspaper story about the range, GG got to see the text the writer had prepared and it was peppered with “Sierras.” Wrong wrong wrong, but the AP style guide said that was right and the author’s editor demanded adherence to the rulebook.
Well, the AP style guide be damned, it is wrong.
It is the Sierra Nevada or the Sierra, not the Sierras, Sierra Nevadas, or (ugh) Sierra Nevada Mountains or (really serious ugh) Sierra Nevada Mountain Range. Part of this is simply parsing the Spanish: Sierra used geographically is a range of mountains, so that last usage is repetitive (“Snowcapped Range Mountain Range”).
So where did this Sierras stuff come from? Part of it was from the early Euroamerican exploration of the range. Many of the routes across the range seem to cross multiple crests (most notably, the very popular Carson Pass route for 49ers had the main Carson Pass and then the higher West Pass, which is a ridge line between two branches of the American River; similar issues seemed present when crossing the main Sierran crest into the Lake Tahoe basin and then out the far side over the Carson Range). So it seemed plausible that there were really multiple ranges within the Sierra. But by the end of the 19th century it was clear that the Sierra really was a single range; that argument was made by Andrew Lawson in 1905 in a talk to the Berkeley Club (later printed nearly unchanged in the University of California Chronicle in 1921):
“In the popular mind the Sierra Nevada is thought of as an aggregate. The term is not plural but the range is referred to as the mountains. The usage constrains the geologist to the same plural reference, but this is only an illustration of a rather common influence of popular usage upon scientific thought, for to him the Sierra Nevada constitutes a magnificent unit, one of the finest examples on the face of the globe of a single range, the type of its class.”
Basically, there is a single range (hence singular Sierra, whereas a group of ranges is Sierras, such as the Sierras Pampeanas in South America); this is true physiographically and structurally. In contrast, the Rocky Mountains (or Rockies, which is acceptable) is made up of numerous ranges (the Tetons, the Elk Mountains, the Gore Range, the Front Range and so on). So to talk of the Sierras is to talk of some other group of ranges. So says the Grumpy Geophysicist.