Complex sentence: The Mosca Pass Toll Road was developed in the 1870s, and stages and the mail route used it regularly through about 1911 when the western portion was damaged in a flash flood.
Split into simpler sentences: The Mosca Pass Toll Road was developed in the 1870s, and stages and the mail route used it regularly through about 1911. That year, the western portion was badly damaged in a flash flood.