Complex sentence: The lone survivor of these mounds is the Sugarloaf Mound, which is located on the west bank of the Mississippi, on the former border between St. Louis and the autonomous city of Carondelet.
Split into simpler sentences: The lone survivor of these mounds is Sugarloaf Mound, which is located on the west bank of the Mississippi. It marked the initial border between St. Louis and the once autonomous city of Carondelet.