Primary or Secondary Sources?

Pony Express Map

Pony Express route map
The Pony Express Route from April 3, 1860 to October 24, 1861. W.H. Jackson map, issued by the American Pioneer Trails Association in commemoration of the Pony Express Centennial from April 3, 1960 to October 24, 1961. The route shows names and locations of relay stations of the Pony Express.
Courtesy of the Geography and Map Division, Library of Congress.

This is a map of the Pony Express route from St. Joseph, Missouri to Sacramento, California. Historians use both primary and secondary sources to help them understand the past. Is this a primary or secondary source? What can you learn about the Pony Express and the American West by looking at this map? Remember to look at all of the details before making a decision.