Complex sentence: They began their journey in Missouri travelling via the Santa Fe Trail and joined the Rose-Baley Party which was the first emigrant party to attempt the final stretch to California via Beale's Wagon Road, at the time little more than a rough trail.
Split into simpler sentences: They began their journey in Missouri travelling via the Santa Fe Trail. They joined the Rose-Baley Party which was the first emigrant party to attempt the final stretch to California via Beale's Wagon Road, at the time little more than a rough trail.