Complex sentence: During his return, Thompson was delayed by an angry group of Peigan natives at Howse Pass which ultimately forced him to seek a new route across the Rocky Mountains through the Athabasca Pass.
Split into simpler sentences: During his return, Thompson was delayed by an angry group of Peigan natives at Howse Pass. He was ultimately forced to seek a new route across the Rocky Mountains and found one through the Athabasca Pass.