Here's what you were given to work with to identify National Park Mystery Spot 13:

To cut to the chase and solve this riddle,
Think "flickering forever in the monarch middle."

Flickering forever refers to an eternal flame. This is a hugely helpful clue. While several landforms in the national parks (cliffs, waterfalls) bear the name Eternal Flame, genuine eternal flames are rare in the National Park System.

Two other words are helpful. Monarch can mean a butterfly species, but is most commonly taken to mean "king." Another word for middle is "center." Putting it together, you get "eternal flame at the King Center." More precisely, it is the eternal flame that burns at the tomb of civil rights martyr Martin Luther King, Jr. and his wife Coretta Scott King in the King Center at Martin Luther King, Jr. National Historic Site in Atlanta.

You'll find a Traveler's Checklist for visiting Martin Luther King, Jr. National Historic Site at this site.

Postscript: The Eternal Light Peace Memorial on Oak Hill at Gettysburg National Military Park has an eternal flame that was first lit by President Franklin Roosevelt on July 3, 1938. An eternal flame is planned for the Flight 93 National Memorial now under construction near Shanksville, Pennsylvania, and scheduled for completion by September 11, 2011.