MLA

Clark, William, 1770-1838. Area of the junction of the Musselshell and Missouri rivers, route about May 20, 1805 [and] Sketches of falls of the Missouri, route about June-July 1805. Box 1, folder 25r. circa 1803-1810. https://collections.library.yale.edu/catalog/2002470.

APA, 6th edition

Clark, William, 1770-1838. (["Box 1, folder 25r"])(circa 1803-1810)[Area of the junction of the Musselshell and Missouri rivers, route about May 20, 1805 [and] Sketches of falls of the Missouri, route about June-July 1805]. https://collections.library.yale.edu/catalog/2002470.