Complex sentence: The Siskiyou Trail was based on Native American footpaths, was expanded by Hudson's Bay Company trappers in the 1830s, and was expanded still further by "Forty-Niners" during the California Gold Rush.
Split into simpler sentences: The Siskiyou Trail followed Native American footpaths, and was extended by Hudson's Bay Company trappers in the 1830s. Its length was increased by "Forty-Niners" during the California Gold Rush.