Complex sentence: The Old Crossing treaty was originally to be consummated at the same spot in September 1862, and a large contingent of Ojibwe chiefs and their family members were encamped at this site when the Sioux Uprising spread to the Red River Valley.
Split into simpler sentences: The Old Crossing treaty was originally to be consummated at this spot in September 1862. A a large contingent of Ojibwe chiefs and their family members actually was encamped at this site when the Sioux Uprising spread to the Red River Valley.