Complex sentence: Slatta (2001) argues that the widespread popularization of Turner's frontier thesis influenced popular histories, motion pictures, and novels, which characterize the West in terms of individualism, frontier violence, and rough justice.
Split into simpler sentences: Slatta (2001) argues that the widespread popularization of Turner's frontier thesis impacted popular histories, motion pictures, and novels. They characterize the West in terms of individualism, frontier violence, and rough justice.