Fur trappers did not journey west into Mexican California. The Rocky Mountains formed a natural barrier to westward expansion until pioneers discovered South Pass in 1824. Before that time, fur trappers focused their efforts in the Rocky Mountain region and the Pacific Northwest, where beaver pelts were more plentiful and valuable. Additionally, Mexican California was under Spanish and later Mexican control until the Mexican-American War (1846-1848), which further restricted access to the region for American fur trappers.