Complex sentence: Unable to cross the Blue Ridge by their horses, they left them with some Indians in the party and climbed them by foot, and sighted the Appalachian Mountains beyond the Shenandoah Valley.
Split into simpler sentences: Unable to cross the Blue Ridge by horseback, they left their mounts with some Indians in the party and climbed the mountains by foot. From the heights, they sighted the Appalachian Mountains beyond the Shenandoah Valley.