Complex sentence: In addition to Michigan's own fires, the Peshtigo firestorm in Wisconsin sent flaming boards of lumber some fourteen miles over the Menominee River and set other fires on the Michigan side of the border.
Split into simpler sentences: In addition to Michigan's own fires, the Peshtigo firestorm in Wisconsin sent flaming boards of lumber some fourteen miles ago. It also crossed the Menominee River and set other fires on the Michigan side of the border.