The CA tartan is based on the Muir Tartan, in honour of John Muir. The Sierra Club has a nice write-up about it on their web site:

http://www.sierraclub.org/john_muir_exhibit/

You'll have to hunt under the "biography", and then under the "Scotland -- Muir's Birthplace" links.

I wanted to see if the National Park Service would "adopt" the Muir Tartan for all Park Rangers to wear, but I never got anywhere with it.

District Tartans are my favourite tartans, even more so than clan tartans.

Cheers!

T.