Possible Clues:

• South Dakota region
• National park in South Dakota ... or, parsed differently, a hint to the meanings and initials of 17-, 24-, 39- and 51-Across
• Nebraska region
• South Dakota's -- National Park
• South Dakota national park
• 1973 film starring Martin Sheen
• Poor and eroded uncultivable region
• Dakota region
• South Dakota's ___ National Park
• Rocky Dakota region