Passage: By the 1850s, Arapaho bands formed two tribes: the Northern Arapaho and Southern Arapaho. Since 1878, the Northern Arapaho have lived with the Eastern Shoshone on the Wind River Reservation in Wyoming and are federally recognized as the Arapahoe Tribe of the Wind River Reservation. The Southern Arapaho live with the Southern Cheyenne in Oklahoma. Together, their members are enrolled as the federally recognized Cheyenne and Arapaho Tribes.

Question: What decade did the Arapaho split?
the 1850s