Complex sentence: The town of Whiteclay in Sheridan County, Nebraska, just over the border from the reservation, was established in the former "Extension" zone and quickly started selling alcohol to the Oglala Sioux.
Split into simpler sentences: The town of Whiteclay in Sheridan County, Nebraska, just over the border from the reservation, was founded in the former "Extension" zone. Merchants and quickly started selling alcohol to the Oglala Sioux.